The approaching the ideal efficiency assumption is possible in Stirling engine. Regenerator is the main component in improving the efficiency of the engine. Besides the Geometry and materials of the regenerator, amount and quality of porosity have significant impacts on the regenerator performance which is focused on this research. The main idea of this study is to evaluate the effect of porosity, or unsymmetrical porosity gradient in pressure drop and the thermodynamic performance of regenerator, so three models of regenerator are developed and analyzed: First, a model in which the porosity is constant and do not change (Common mode). In the Second model, the length of regenerator porosity is changed from high to low and in the third model, the length of regenerator porosity is changed from low to high. All versions of models have the same global porosity. Each of these models is separately simulated in Ansys software and validated with experimental results from a laboratory sample. The results show that in the second model with a small increase in pressure drop during recovery, the amount of energy recovery increase dramatically compared to the conventional model.