Gallium oxide (β-Ga₂O₃) exhibits an ultra-wide bandgap (E_g=4.5~4.9 eV) and a high critical breakdown electric field (E_br=8 MV/cm). The Baliga's figure of merit for β-Ga₂O₃-based devices is theoretically approximate four times and ten times as large as that of SiC- and GaN-based devices, respectively. Nevertheless, the breakdown voltage of β-Ga₂O₃ power devices is considerably below the theoretical limit; and there are few studies on high-power devices and their thermal stability. In addition, the low thermal conductivity of β-Ga₂O₃ materials and the presence of multiple defects result in many reliability issues, including the shift of electrical characteristics and the accelerated degradation of device performance. First, this work presents our researches on novel structures of β-Ga₂O₃ power devices, including the analysis of experimental results measured from the fabricated devices and the investigation of their electro-thermal characteristics. Second, this work studies the electro-thermal reliability of β-Ga₂O₃ mental-oxide-semiconductor field-effect transistor (MOSFET) and heterojunction field-effect transistor (HJFET). The ionized traps model and interface dipole ionization model are proposed to explain the degradation mechanism of β-Ga₂O₃ MOSFET and HJFET. Additionally, a novel reliability reinforcement technology is proposed to enhance the electro-thermal reliability of β-Ga₂O₃ HJFET. These studies indicate the considerable potential of β-Ga₂O₃ power devices in high-voltage, low-loss and high-reliability applications. Further, these studies provide novel insights into the design and optimisation of β-Ga₂O₃ power devices, and effectively advance the practical development of β-Ga₂O₃ power devices.