© 2022 American Institute of Chemical Engineers.Stirling engines take attention due to their high thermal efficiency potential. Many experimental and numerical studies have been performed up to date. Numerical performance analyses of Stirling engines implemented with CFD simulations are classified as 4th order analyses. In the present study, the effect of compression ratio on engine performance of a beta type Stirling engine with rhombic mechanism was investigated by 2D CFD simulation. CFD simulations of the Stirling engine were performed by ANSYS Fluent software. The engine examined by CFD analysis was modeled using the dimensions of an engine manufactured and tested in a previous study, and validation of the CFD simulation was conducted regarding in-cylinder pressure at 4 bar charge pressure. The crank radius and length of the regenerator zone of the engine were optimized by considering the compression ratio of the engine and engine performance was investigated. Maximum engine performance was obtained at a compression ratio of 2.24 which was ensured by 31.5 mm crank radius and 609.9 mm a length of the regenerator zone. By optimizing the dimensions the engine performance was improved by 99% compared to the base engine configuration. The cyclic work, power, torque, and thermal efficiency were calculated as 62.47 J, 728.93 W, 9.94 Nm, and 37.7% respectively.