The feasibility of real-time monitoring in laser induced interstitial thermotherapy (LITT) of biological tissues using near infrared spectrum technique is explored and validated. In vitro liver and rats with subcutantaneous implanted tumor models were chosen as experiment objects. Changes of absorption coefficient (μa) of tissue are real-time measured by near infrared spectrum system in laser induced interstitial thermotherapy. The result indicates that μa has some variations during LITT. The change law of μa invitro livers experiments is significant. It slowly goes up under low power, and rises greatly based on high power when biological tissues reach certain temperature(around 50°C). The change laws of μa during tumor experiments is different according to characteristics of tumor tissue which can be used for future clinical treatment. Experimental results show that μa has valuable reference as a real-time monitoring factor in LITT of entity organizations, but it also needs further study as evaluating factor of the tumor treatment appraisal.