A theoretical consideration of two‐dimensional ac temperature waves in a two‐layered platelike sample for application to thermal diffusivity measurement parallel to a sample surface using an ac calorimetric method is presented. The analysis is based on the Fourier‐transformed Green’s function technique. It is concluded that this thermal system deviates from an apparent one‐layered system of which the thermal diffusivity parallel to a sample surface is calculated based on steady state heat flow under the following conditions: (a) the frequency of the supplied ac thermal energy is high, (b) the difference between the thermal diffusivities of the layers is large, and (c) the layer with higher thermal diffusivity is thin. This deviation is due to the inhomogeneous ac temperature distribution normal to the sample surface. If experimental conditions are the reverse of conditions (a)–(c), an ac calorimetric method can be used to measure the thermal diffusivity of thin samples on substrates. ©1996 American Institute of Physics.