In the paper a method for correction of heating non-homogeneity applied in defect detection with the use of active thermography is presented. In the method an approximation of thermal background with second- and third-order surfaces was used, what made it possible to remove partially the background. In the paper the simulation results obtained with the abovementioned method are presented. An analysis of the influence of correction of heating non-homogeneity on the effectiveness of defect detection is also carried out. The simulations are carried out for thermograms obtained on the basis of experiments on a test sample with simulated defects, made of a material of low thermal diffusivity.