An advanced ac calorimetric method to measure thermal diffusivity of a thin sample is developed by using an optical reflectivity technique. A modulated infrared laser is used to heat the front surface ofa foil specimen. The reflectivity of a continuous-wave He-Ne laser at the rear surface of the specimen is detected by a photoreceiver.According to the temperature dependence of reflectivity, the ac temperature response is obtained. Thermal diffusivity is deduced from a one-dimensional heat conduction equation. A stainless-steel foil and a copper foil are chosen as the samples. The present results agree well with the data in the literature.