In this paper, equations of motion for a horizontal axis wind turbine with movable base are extracted and natural frequencies and vibration of the system is studied. The wind turbine tower is assumed rigid while its blades are modeled as flexible beams. In-plane bending and twisting are considered as two degrees of freedom for blades motion.The shaft connected the tower to blades is assumed rigid and its rotational velocity is considered.In this paper, specifically, a 5-megawattfloating horizontal axis wind turbine, which it’s basehas three angular velocities in different directions,is studied.Due to the complex shape and variation of the properties along the length, the turbine blade properties such as mass and geometric parameters are extracted by curve fitting in MATLAB.The equations of motion and boundary conditions are derived by Hamilton's principle and then are transformed to ordinary differential equations by Galerkin method. By setting the governing equations to standard form (space state), eigenvalues and frequencies are calculated. The numerical results are compared with published results and good agreement is observed.Then the effect of various parameters on turbine blades frequencies and time responses are demonstrated. Results show that the tower base angular velocity and blades rotational speed have considerable effects on turbine blades time response and vibration frequencies.