Based on the columnar to equiaxed transition (CET) map, the geometrical relationship for the cross section of the molten pool is established and the relationship between the solidification condition and the microstructure for Ni-based single crystal superalloys fabricated by selective laser melting (SLM) is obtained. The SLM experiments are conducted on the (100) crystallographic plane of SRR99 single crystal substrate along the ［001］ direction. The results show that the minimum angle between the solidification velocity and the laser scanning velocity is about 46.4° (larger than 45°) under the conditions of laser power of 160 W, powder-layer thickness of 20 μm and scanning speed of Vb≤30 m·min-1. It can be predicted that the microstructure of the laser molten pool is composed of ［001］ dendritic crystals growing from bottom to top and ［010］ dendritic crystals on two sides. The experimental results coincide well with the theoretical analysis results. In addition, good metallurgical bonds are formed between layers and the crystallographic orientation of substrates gets extended.