A detailed numerical simulation of the performance of thermal module having "thermal via" is made in this study. The results indicate the temperature distributions from the numerical simulation are significantly affected by spreading resistance. The filled thermal via can considerably improve the performance of thermal module. For further explanation of the significant drop of junction and thermal resistance at sub-mount with thermal via, the detailed thermal resistances distribution in the thermal module are further examined. It is found that the significant drop of thermal resistance is mainly from package level with the help of thermal via filled in submount. However, the effect of PCB on the thermal resistance is quite different for the simulated geometries, a "maximum" ratio of thermal resistance of PCB had occurred. The thermal resistance of heat sink remains the same for all simulated case. With further adding the thermal via, the effect of heat sink on the overall resistance will become more and more pronounced.