Supersonic inlet has a key role in the performance of air-breathing engine special in ramjet engines. The function of a ramjet engines diffuser is to decelerate the air velocity from its free-stream at the intake to a velocity which is compatible with the available flame velocity. Accurate design of inlet has an important effect on the performance of a ramjet system and its main parameters. In this study, an external compression supersonic inlet designed at Mach 3. Flow behavior for this type of inlet is more complicated than the two-dimensional inlet. Because of this character, a computer program for solving the equations of supersonic flow through the cone, and the relationship between the parameters and the principles of the inlet has developed which integrates the final geometry, including center body geometrical details, inlet cowling and subsonic channel as . Since viscous forces cannot be ignored because they cause complex shock patterns and flow separation, the simulation and flow behavior pattern is modeled and validated by computational fluid dynamics software. For this purpose, the RNG K-ɛ model was used for flow turbulence modeling and second-order accuracy was used for of convection term. The results show that the designed geometry can meet the desired performance requirements.