The present paper is performed to investigate the effect of working fluids on a double-acting Stirling engine performance. A numerical model that includes major thermal losses that decrease the Stirling output power and efficiency was developed. The influences of tree monatomic gases (helium, Neon, and Argon) and one diatomic gas (nitrogen) on optimal compression ratio and optimal regenerator form factor, as well as the optimal frequency of the Stirling engine, are studied. It was found that the optimal design of the regenerator, allowing the maximum output power, depends on the working fluid characteristics. Results showed that the compression ratio corresponding to the optimum power depends on the working fluid used. It was shown that to achieve a high output power of the Stirling engine, and it is preferable to use a monoatomic gas having the highest heat capacity and thermal conductivity and the lowest density as working fluid