Melting and re-solidification of the substrate plays an important role in thermal spray coating. A good understanding of this phenomenon will help us to achieve better bonding. A numerical model is developed to investigate the solidification of the droplet, and melting and re-solidification of the substrate. The solidification interface movement is obtained by applying a rapid solidification model on the solid/melt interface. Numerical simulations have been used to study the influence of materials and temperatures of the splat and substrate on substrate melting and re-solidification. In the corresponding experiments, the molybdenum powder is sprayed onto a stainless steel, brass (70%Cu) or aluminum substrate by atmospheric plasma spraying system. The crater depth of the substrates has been measured. Experimental results show that the material properties of the splat and substrate and melting temperature of the substrate play important roles on substrate melting and maximum melting depth. A dimensionless parameter, temperature factor, has been proposed from analysis and can be used as an indicator whether a substrate melting will occur for a certain combination of the droplet and substrate, and this parameter can be correlated with the maximum melting depth of the substrate.