Abstract: This paper presents a method for double nonlinear analysis of the loading capacity of oil derrick steel structures. A spatial beam element is adopted in the nonlinear analysis. The large deformation of the structure is considered by constructing the incremental tangent stiffness matrix which consists of both geometric stiffness matrix and defamation matrix. The material nonlinearity is treated using the concentrated inelasticity approach. The ultimate load of the structure is tracked using the arc-length method. A numerical example of an A-type oil derrick is presented to verify the present method.