Rotary Wankel expander has advantages compared with other expanders due to its compactness, low vibration, noise and cost. This study aims to enhance the performance of rotary Wankel expander device using two stages to benefit from exit flow from the first stage as the inlet to the second stage for extra power output. Computational fluid dynamics (CFD) was used to investigate the effect of the two stages configurations on the expander's power output and isentropic efficiency using ANSYS/ Fluent 16.2. Single stage CFD-3D results were compared to published work showing close agreement. The CFD modelling was used to investigate the performance of the rotary device using various Wankel expander sizes (r = 48, e = 6.6, b = 32) mm and (r = 58, e = 8, b = 40) mm both as single stage and as two stage expanders with different configurations. Results showed that combining the two Wankel expanders horizontally with the larger one at front produced 8.52 kW compared with single stage which gives 4.75 kW power output at the same initial conditions. It was also seen that increasing the inlet pressure produced an increase in the power output. Furthermore, the maximum isentropic efficiency can be achieved to reach 91 % with the inlet pressure 6 bars and inlet temperature 400 K at 7500 rpm for two stages with different size.