Laser induced thermotherapy (LITT) is a promising local treatment for irresectable liver tumors. For evaluating the effect of laser application in real time and ensuring the optimal effect in LITT, exploration the changes of optical properties and temperature distribution in tissue, and their correlation are indispensable. 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. Fresh porcine liver tissue samples in vitro were examined in different laser doses (0.95W, 1.23W, 1.42W) and at a certain heating time (600s). 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 they rose faster when the laser power was greater. When the tissue temperature was below 50°C or above 65°C, there was only a slight increasing of μ’s .When the tissue temperature in the range of 50°C to 65°C, μ’s changed more obviously, and there was a clear linear relationship between μ’s and tissue temperature. The linear equation was obtained for the first time. These results can effectively guide the study of evaluating the effect in real time during LITT.