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The flap-lag coupled oscillation of a two degrees of freedom wind turbine blade is studied in this paper.A blade experiences feather-flap-lag vibration due to wind.Considering the blade as a flexible-body system, the simplified two degrees of freedom model is derived.Selecting two modes functions with respect to flap and lag respectively and using Galerkin method, two coupled ordinary differential equations are obtained.A linear transformation is used to uncouple the linear part of these equations.Another set of equations which have uncoupled linear part is derived.The ratio of associated frequencies is assumed to be in almost 1∶2, so internal resonance occurs for lag-flap.The multiple scales method is used to get a set of three amplitude equations whose coefficients are functions of wind velocity.From these amplitude equations, four branches of equilibriums path are obtained and bifurcation analysis of these branches is given.