Considering the different physical properties between the tumor and the surrounding normal tissue, a two layer model was developed to describe the energy transport and the temperature behavior in the tumor tissue during the laser induced interstitial thermotherapy (LITT). Photon transport was simulated with the Monte Carlo method and then energy distribution was used to calculate the temperature distribution with Pennes bio heat equation. Both the time required to coagulate the tumor and the thermal damage depth are numerically predicted.