As the first step of developing numerical model for Stirling engine, numerical studies have been performed to investigate the oscillating flow and heat transfer in an isolated gas spring. The compressible Navier-Stokes equations are used to model the effects of oscillating fluid flow and pressure level on heat transfer and work. The gas spring modeled here has low Mach numbers and is sufficiently small that the time required for acoustics to propagate across them is negligible. Moving grid technology to simulate the piston motion in gas spring was also applied here to simulate the movement of the piston. The numerical results include temperature filed, velocity vector and pressure variation during the cycles. Numerical results were also compared with the reference data non-dimensional hysteresis loss for the gas spring over an operating range with other CFD code simulation results and experimental results. The comparisons show very good agreement between simulation results and other results. This study had shown that the CFD codes used here could predict the flow and heat transfer under oscillatory condition.