The paper presents the method for determining the subsurface defects in material by means of active thermography and 3D numerical non-stationary heat conduction model. Determination of the subsurface defects by Thermal Non- Destructive Testing means the description of the defects dimensions and its location. The method is tested on the samples made of metal and composite materials having known subsurface defects. The results obtained are discussed from the point of view of influence of the neighboring defects on the current contrast curve and possibilities of predicting the behavior of the composite material by estimation of their thermal properties. The software developed enables the calculation of the temperature distribution through the sample versus time and location of the defect by means of inverse method involving experimentally obtained thermogram.