The method to investigation the changes of optical properties and evaluating the effect of laser application in real time is presented in this paper. In vivo measurements of the reduced scattering coefficient (μ’s) and tissue temperature (T) were performed with a functional near infrared spectroscopy (fNIRS) system and a temperature measurement system during LITT. Mice liver tumors were examined in vivo in different laser doses (0.8W, 1.0W, 1.2W, 1.4W) at a certain heating time (600s). The results showed that there were the same rising tendencies of the reduced scattering coefficient and temperature during LITT. They increased quickly at the beginning, gradually reached a stable state, and rose faster when the laser power was greater. When the tissue temperature was in the range of 50°C to 70°C, there was a clear linear relationship between μ’s and T. The linear equation was obtained for the first time.