A three-dimensional compressible CFD code has been developed to study the heat transfer characteristics of a twin-power piston gamma-type Stirling engine. The results include temperature contours, velocity vectors, and distributions of local heat flux along solid boundaries at several important time steps as well as variation of average temperatures, integrated rates of heat input, heat output, and engine power. It is found that Impingement is the major heat transfer mechanism in the expansion and compression chambers, and the temperature distribution is highly non-uniform across the engine volume at any given moment. This study sheds light into the complex heat transfer mechanism inside the Stirling engine and is very helpful to the understanding of the fundamental process of the engine cycle. (C) 2014 Elsevier Ltd. All rights reserved.