The aim of this study is to improve the power output of HAWT by using the dual rotor wind turbine. DRWT is consisting of a primary upwind rotor called front rotor, which has a diameter 0.9 m and using NREL S826 airfoil. An auxiliary downwind rotor called rear rotor using the same airfoil. The study is conducted by using computational fluid dynamics (CFD). The major goal of the research is to optimize the DRWT performance by adjusting two parameters: the diameter ratio of two rotors and the axial distance between them. In terms of the diameter ratio, there are three approaches. In the first one (DR=1.5), the rear rotor diameter is smaller than the front rotor. In the second one (DR=1), the two rotors have the same diameters. In the third one (DR=0.75), the rear rotor diameter is bigger than the front rotor diameter. The front rotor diameter is kept constant while the rear rotor diameter is changed. For the axial distance, used are as follow (0.25D1, 0.5D1, 0.75D1 and 1D1). The numerical model is performed by using k-ω SST Turbulence model. The results show that the axial distance parameter makes a little change in Power output. Using the model of DR=0.75 increases the CP by 13.3% at λ=5 with respect to the baseline model. Testing the model of DR=1.5 doesn't show sensible improvement while using the model of DR=1 increases the CP by 7.8% at λ=5.