Reliable thermophysical properties of submicrometer thin films are necessary for thermal design of advanced devices such as high density optical disks or highly integrated semiconductor devices. In order to measure thermal diffusivities of thin films thinner than 1 micrometer, a picosecond thermoreflectance measurement system has been developed. A film face of a transparent substrate side is heated by picosecond laser pulses and the temperature change on the front face opposite to the heated area is probed by the reflected intensity of other picosecond laser pulses. The heat diffusion across the thin film can be observed directly by this method. The thermal diffusivity of the thin film is calculated from the heat diffusion time across the thin film and the thickness.