Cycloidal reducer, commonly used in industry for decades, is a speed reduction device with high speed ratio, high efficiency, and high locating precision. This thesis studies the force on a cycloidal speed reducer with midified teeth. Firstly the cycloidal gear profile is generated by using theory of gearing. To satisfy engineering requirements, the cycloidal gear profile is then modified. Second, the force performance and kinematic error are analyzed with the results of tooth contact analysis. Finally, an optimization design is discussed. The optimal results of tooth modification parameters are designed with the optimization processes.