The nonlinear finite element model of a 100 kW wind turbine blade was built to simulate the composite structural responses. According to the GL guidelines, ultimate loads in 4 directions were applied to the blade model to analyze the strain and safety factor distributions, and the linear and nonlinear buckling behaviors. It is found that under the condition of an extreme flap-wise load, a large strain happens in the middle of the blade and the transition between the blade root and the maximum chord section; the first eigenvalue and nonlinear bucklings occur around the blade tip; the nonlinear buckling analysis is more conservative than the linear analysis. Under the conditions of extreme edge-wise loads, a large strain exists at the trailing edge of the middle of the blade, and the first eigenvalue and nonlinear bucklings occur around the trailing and leading edges of the maximum chord section; in this case, the linear buckling analysis is more conservative than the nonlinear analysis. The first eigenvalue buckling load factors are larger than the nonlinear buckling load facors under the limit load conditions in all 4 directions.