Microwave ablation (MWA) status monitoring in real time plays a key role in assessment of therapeutic effectiveness. As a novel real-time assessment method, near infrared spectroscopy (NIRs) was used to evaluate the ablation efficacy. MWA experiments were carried out on in vitro porcine livers. An optical measurement system for biological tissue is developed by our lab to monitor reduced scattering coefficient (μ^'_s) at 690 nm of the coagulation zones. It is noted that μ^'_s of liver tissue, which increases as the liver tissue being ablated, is clearly related with the coagulation status. μ^'_s of normal tissue and coagulated tissue is 3–5 and 17–19 cm^-1, respectively. Continuous changes of μ^'_s demonstrate that optical parameter can be used as an efficacy evaluation factor because it essentially indicates the degree of thermal damage. Compared with temperature, optical parameter is more sensitive and accurate, which is promising for real-time therapeutic efficacy assessment in MWA.