In industry, especially in the high technology sector such as aerospace, we produce and we use increasingly new materials for the construction of new structures that have good thermal and mechanical properties. The characterization of these materials requires knowledge of their thermo-physical properties. Thermal diffusivity is an important parameter in the materials characterization. Lock-in thermography is widely used in the materials thermal characterization. It involves applying on the front face sample a heater in the form of a sine wave and analyzing the phase difference or the amplitude difference between the incident thermal wave and the transmitted thermal wave. Indeed, the passage of the thermal wave through a material is influenced by its thermal diffusivity. We used the finite element method, in three dimensions, to calculate the instantaneous temperatures of the front and rear faces of the inspected sample, and deduct their phase shifts and therefore the sample thermal diffusivity. Our contribution in the lock-in thermography technique is the development of a new model for the thermal diffusivity evaluation with good precision. The results for polystyrene are very satisfactory. Indeed, the thermal diffusivity calculated by our new model is very close to the value reported in the literature. The proposed new model can be used in the characterization of new materials.