With the development of electronic power devices towards high voltage, high current and high power density, they will generate more heat and bear greater thermal stress during the operation, which puts forward higher requirements for the heat dissipation ability and reliability of ceramic substrates used for devices. Conventional ceramic substrates such as alumina (i.e., low thermal conductivity, only 20-30 W·m^𕒵·K^𕒵) and aluminum nitride (i.e., poor mechanical property, such as bending strength of 300 MPa and fracture toughness of 3-4 MPa·m^1/2) are difficult to meet the requirements of high power devices. Silicon nitride (Si₃N₄) ceramic becomes an ideal ceramic substrate for high power devices due to the high theoretical thermal conductivity and excellent mechanical properties. However, there is still a significant gap between the actual and theoretical values of thermal conductivity for Si₃N₄ ceramics, and the lattice oxygen content is a main affecting factor. Moreover, suitable sintering additives need to be added during the preparation of Si₃N₄ ceramics, and the microstructure (i.e., density, grain morphology, grain boundary phase content and distribution, and average thickness of grain boundary film) changes due to the introduction of sintering additives, which affects the thermal conductivity of Si₃N₄ ceramics. The microstructure evolution of Si₃N₄ ceramics is also closely related to the forming technology and sintering system. Therefore, the optimization of preparation processes of Si₃N₄ ceramics has a positive effect on its thermal conductivity improvement.In the review, the microstructures and thermal properties of Si₃N₄ ceramics were introduced. Some influencing factors of the thermal conductivity of Si₃N₄ ceramics were discussed and the related impact mechanism were illustrated. The α-Si₃N₄ phase is metastable and transforms to β-Si₃N₄ phase through the dissolution precipitation reaction at a high temperature. A lattice reconstruction also occurs at the same time. The β-Si₃N₄ phase exhibits a high ideal thermal conductivity. The actual thermal conductivity of Si₃N₄ ceramics deteriorates gradually with increasing the lattice oxygen and grain boundary phase contents. Similarly, the continuous-distributed grain boundary phase is also harmful to the thermal conductivity of Si₃N₄ ceramics. These existed defects and microstructures lead to increased phonon scattering during the transmission process, which does not favor the thermal conductivity improvement. The large elongated β-Si₃N₄ grains are beneficent to the thermal conductivity enhancement because of the decreased phonon scattering.The ways to improve the thermal conductivity of Si₃N₄ ceramics are to regulate their lattice oxygen and microstructures. In the review, the lattice oxygen regulation role, the microstructure evolution process and the thermal conductivity enhancement mechanism of Si₃N₄ ceramics could be elaborated from four aspects, i.e., raw material powder, sintering additives, sintering process and structural texture. The introduction of oxygen impurity content could be reduced with high purity silicon nitride powder as a raw material and the ball milling process optimization. And the microstructure and lattice oxygen content can be regulated via selecting rare-earth oxides with small ion radius and non-oxides as sintering additives and optimizing the sintering process (i.e., sintering temperature, soaking time, sintering atmosphere and so on), which are beneficial to promoting the thermal conductivity. In addition, the high thermal conductivity along the direction of Si₃N₄ grains orientation can be also obtained through a structural texture technology.The development status and challenges of high thermal conductive Si₃N₄ ceramics were discussed. The routes to realize the localization of high thermal conductive Si₃N₄ ceramics are to strengthen the source control of raw material production, thus improving the performance of production equipment and establishing the evaluation standard system of product performance.Summary and prospectsThe Si₃N₄ ceramic is an ideal substrate material for high power device, and its high thermal conductivity is essential. However, there is still a significant gap between the actual and theoretical values of the thermal conductivity of Si₃N₄ ceramics due to the influence of lattice oxygen impurity and microstructure composition. The approaches to obtain Si₃N₄ ceramics with a high thermal conductivity are to decrease the lattice oxygen content, reduce the grain boundary phase and obtain the double peak structure composed of fine grains and a high aspect ratio β-Si₃N₄ grains through improving the purity of raw materials, selecting appropriate sintering aids and forming methods and optimizing the sintering system.To obtain high thermal conductive Si₃N₄ ceramics, a research on the raw material powder with a high purity and a low oxygen content should be strengthened. The high quality equipment for Si₃N₄ ceramics production should be developed through controlling the sintering and processing technology. The practical application research should be promoted continuously and the completed application closed loop should be formed among powder production companies, product manufacturing enterprises and users. The timely feedback and response about the problems discovered during this process should be provided quickly to improve the performance of high thermal conductive Si₃N₄ ceramic products.