The laser-flash method (LFM) is a technique commonly used to measure thermal diffusivity of homogeneous and isotropic materials, but can also be applied to macroscopically inhomogeneous materials, such as composites. When composites present thermal anisotropy, as fiber-reinforced, LFM can be used to measure the effective thermal diffusivity (αeff) in the direction of heat flux. In this work, the thermal behavior of composites during thermal diffusivity measurements with the LFM was simulated with a finite element model (FEM) using a commercial software. Three composite structures were considered: sandwich layered (layers arranged in series or parallel), fiber-reinforced composites, and particle composite (spheres). Numerical data were processed through a nonlinear least-square fitting (NL-LSF) to obtain the effective thermal diffusivity of the composite. This value has the meaning of “dynamic effective thermal diffusivity.” Afterward, the effective thermal conductivity (λeff) is calculated from the dynamic effective thermal diffusivity, equivalent heat capacity, and density of the composite. The results of this methodology are compared with the analytically calculated values of the same quantity. This last assumes the meaning of “static effective thermal conductivity.” The comparison of the dynamic and static property values is so related to the inhomogeneity of the samples, and a deviation of the temperature versus time trend from the analytical solution for the perfectly homogeneous sample gives information about the lack of uniformity of the sample.