Modeling and analysis of inventory systems in reverse logistics is more complex than in forwards logistics, because in reverse logistics not only amount of demand is not clear, but also uncertainty of product return is appeared in the system. In this paper, an inventory system with the possibility of product return is modeled by means of simulation tools and then thermal equivalent of inventory model, using laws of heat transfer is developed. In order to provide the thermal equivalent, components of the inventory system are known and for each component the thermal equivalent is introduced and then sensitivity analysis is used to show the similar behavior of the two models. Hence, analyzing the thermal model can lead us to know the effect of different policies and parameters on inventory system performance. Thermal equivalent model presented in this paper is a strong base for inventory system analysis with more complex structures in future studies.