Objective To explore optical property changes of biological tissues during thermal denaturation. Methods Dog livers, dog kidneys, dog muscles and rat muscles were used as four kinds of samples in the experiment. The integrating-sphere system measured intensity changes of collimated transmission light, diffuse transmission light and reflection light, when samples were heated from 30℃ to 100℃ by a hot air gun at 650nm laser irradiation. The curves of absorption coefficient, scattering coefficient and scattering anisotropy factor were calculated with the Kubelka-Munk model. Results Optical properties of tissues were affected by temperature. The transmittance and scattering anisotropy factor of samples decreased, while absorption and scattering coefficients increased during thermal denaturation. Conclusions Thermal denaturation of tissues cause transmission weakened, absorption and scattering enhanced.