In the present work, viscous, incompressible and turbulent flow around a 5-blade-propeller, worked behind an underwater body is simulated using sliding mesh with unsteady flow consideration. The governing equations are solved by means of a commercial code, which is a general CFD software, using finite volume methods. Also a proper algorithm is used to obtain advance ratio of propeller working behind the body. The unstructured grid generated around the submarine and propeller includes17,000,000tetrahedral cells with 12 boundary layer meshes. Also flow around the propeller in openwater condition and around the body without propeller is simulated in order to compare results. The unstructured grid around the propeller in openwater condition includes2,100,000and around the body includes 17,500,000 tetrahedral cells with 12 boundary layer meshes. In order to validate this work, flow around the submarine and propeller B series model is simulated separately. The hydrodynamic forces are calculated and showed a good agreement with the experimental. In body simulation, the hydrodynamic coefficients of the forces acting on the body is estimated for submarine speed ranging from 1m/s to 8.24 m/s and flow angle of attack is zero. It shows that submarine drag coefficient decreases when Reynolds number increases. Investigation of flow in stern region of the submarine shows that fraction wake has a 15% reduction with increasing submarine velocity. Simulation of the propeller in openwater condition shows that the thrust and torque coefficient acting on the propeller decrease with increasing advance ratio. Investigation of flow in upstream region of the propeller shows that induced velocity of propeller decreases with increasing the advance ratio. Simulation of the propeller when working behind the submarine shows that the low magnitude of trust and torque of each blade occur 4 times in 1 rotation of propeller and it is due to high axial velocity influenced by stern wings. Although 4 stern wings make symmetrical wake field at the propeller plane, wake of sail causes the effective wake field and fluctuating of blade forces to be disordered. Result shows that the thrust and torque coefficient acting on the propeller working behind the body is less than the thrust and torque coefficient on the propeller in openwater condition at the same advance ratio. Results also show that the maximum efficiency in propeller working behind the body corresponds to a lower value of advance ratio compared to a uniform inlet velocity distribution. The hydrodynamic coefficient of the forces acting on the body with the propeller working behind is more than the hydrodynamic coefficient of the body without the propeller at the same Reynolds number. Key Words : Submarine, Marine propeller, Unsteady flow, CFD, Body and propeller interaction.