To investigate the feasibility of in vivo effect assessment monitoring by functional near infrared spectroscopy (FNIRS) during laser induced interstitial thermotherapy (LITT) in real time, the reduced scattering coefficient during LITT therapy by different laser powers and heating time on Fresh pork liver in vitro and the subcutaneous implanted rat liver cancers by FNIRS system are recorded in vivo. The reduced scattering coefficient (s）which gets through the FNIRS increases during LITT, it increases quickly at beginning, and gradually reaches the stable state. s rises faster when the laser power is greater. There is the same changing trends of s of different tissues, but there is difference between the curve shapes. The system of FNIRS can be used to assess the therapy effect during laser induced interstitial thermotherapy in vivo, and s can be used as a new effect assessment factor. By monitoring the changes of s can effectively guide clinical therapy.