One of the most important devices in the basic sciences, medicine, industrial, and agriculture is the particle accelerators.Nowadays, circular accelerators such as the baby cyclotrons are extensivelyused in many different fields, e.g. in medical sciences. The cavity of the babycyclotron plays an astonishing role in the acceleration of the chargedparticles. Appling large electric fields, the acceleration occurs in the gapsof the cavity Dee’s. In the present research, the cavity of a baby cyclotron at72 GHz, which could be installed in the hospitals to produce specialradioisotopes, is studied. So, the electric field and scattering matrix of thecavity are simulated with the use of the Computer simulation Technology (CST)software. Using the CST software as well as the CYCLON code, the coupler anddynamics of the particle beam are also perused, respectively. By changing thedimensions of different parts of the cavity, the optimized version of thecavity is achieved, for which the returning radio frequency power is low enoughto efficiently accelerate protons up to 20 mega electron volts.