Modulated photothermal techniques provide useful methods based on linear relations between measurable quantities to obtain the thermal diffusivity of homogeneous materials. In this letter the applicability of such linear relations in the case of unidirectional fiber-reinforced composites (UFRC) is analyzed. Based on the solution of the propagation of plane thermal waves in a material with aligned buried cylinders, we found that the linear relation valid for homogeneous materials still holds for UFRC. However, its slope does not give the effective thermal diffusivity as derived from the effective thermal conductivity measured by steady-state experiments. Anyway, as the number of cylinder rows increases the thermal diffusivity measured by modulated methods approaches the steady value asymptotically. The model is validated by photothermal measurements performed on calibrated samples.