Among the marine propulsion systems Contra-Rotating Propellers (CRP) are one of the systems with highest efficiency. The Contra-Rotating Propellers principle, comprising two coaxial propellers sited one behind the other and rotating in opposite directions.Because of two propeller configuration, Contra-Rotating Propellers posses a capability for balancing the torque reaction from the propulsur which is an important matter for torpedo and other similar propulsion problems.In this study, flow around a Contra-Rotating Propellers system is simulated using FLUENT code and then its hydrodynamic performance is investigated. This investigation contains three parts that in each one of those effect of a specific parameter on the hydrodynamics of a Contra-Rotating Propellers system is investigated. First, flow around a Contra-Rotating Propellers system with the same geometry and angular velocity for both of fore and aft propellers are investigated and hydrodynamics performances including efficiency, thrust and torque coefficients are obtained at a wide range of advance ratios. The flow simulation was done for two similar five-bladed propellers. The diameters of both propellers were 2 meters and several different spaces between the two propellers from 0.2D (Diameter) to 1D were selected. At the second part, the effect of the rotaion ratio of two propellers on the hydrodynamics performance is investigated. In the both cases, simulations were done at the open water conditions. In this condition calculation is done based on a uniform inlet velocity.Finally, hydrodynamics performance of the Contra-Rotating Propellers system in open water condition is compared with the results obtained when the Contra-Rotating Propellers system was located behind hull of the submarine. In this case, due to effect of hull and control surfaces of submarine, inlet flow to the propeller system was non- uniform.Results show that by reducing the distance between two peopellers, a slitgh reduction in thrust and efficiency was observed but by more distance decreasing these coefficients were increased. Also, the thrust distribution indicated that the lower %20 of each blade has the minimum portion and the upper %40 has the maximum. In addition, by increasing the rotation ratio, the suction effect due to the aft propeller on the fore propeller increases. Also, by increasing the rotation ratio disturbance will shift to the lower advanced ratios.In non- uniform case, investigations showed that the hydrodynamics coefficients are higher than those were obtained for open water condition, provided the applied velocity for open water condition equals with the submarine velocity. Key words: Contra- Rotating Propellers, Hydrpdynamics performance, Open water, Non- uniform, Distance, Rotation ratio