18Ni300 maraging steel was formed by selective laser melting(SLM)technology, the microstructure and mechanical properties of the deposited and heat treated states were investigated of the samples fabricated by SLM with 18Ni300 raw powder material. The results showed that the evolution mechanism of cell-columnar dendrites from the bottom to top of the micro-melting pool of the as-built sample is determined by the ratio of the temperature gradient to the solidification rate (G/Vs) at the front of the solid-liquid interface. However, the mutual transition of the hexagonal cell, the elongated cell and the strip structure existing in the microstructure is accomplished by geometric topology transformation. After heat treatment, the cell, columnar and sub-micron structures in the microstructure of the as-built sample disappeared, and precipitates fine lath martensite with fine granular precipitate. The mechanical properties test showed that the Vickers micro-hardness and tensile strength increased significantly after solution aging heat treatment, but the elongation decreased. At the same time, the fracture morphology presented a typical ductile fracture.