In this paper, the principle and experiment of the transient thermal gratings method are introduced. To demonstrate the applicability of the method, the thermal diffusivities of semiconductors, metals, and alloys have been measured by transient reflection grating method. As a result, the measured thermal diffusivities agree well with the values of literatures. In addition, the thermal diffusivities of several alloy samples have been analyzed, which illustrate that different alloy structures have different thermal diffusivity mechanisms. Furthermore, interfacial acoustics wave in the air above the sample′s surface has been observed in the experiments.