This study presents an eigenvalue analysis of wind turbine system to determine modal frequencies, damping ratios and mode coupling at various wind speeds and to seek potential flutter instability. A comprehensive dynamic response analysis is also performed with different considerations of wind loads on turbine blades and tower. The results show that wind load on tower has a negligible effect on blade response of both operational and parked turbines, and on tower response of operational turbine, but has a significant influence on tower response of parked turbine. The tower extreme responses due to wind loads on blades and tower can be estimated separately and then combined for the estimation of total extreme response. In current wind turbine design practice, the tower extreme response due to wind loads on blades is often calculated using an equivalent static load modeling in terms of concentrated force and moment at the tower top. The combinations of the tower extreme responses are implemented under the assumption of full correlation. This study presents an improved equivalent static load model with additional distributed inertial force on tower, and introduces the square-root-of-sum-square combination rule, which are proved to provide a better prediction of tower response.