Abstract: A nonlinear finite element computational model for reinforced and prestressed concrete slender columns with arbitrary cross-section under axial load and biaxial bending is presented. The effects of both geometrical nonlinearity and material nonlinearity are considered. In this paper, geometrical nonlinearity is caused by element deformation and secondary moment of axial force, and material nonlinearity is caused by nonlinear stress-strain relations of materials and section stiffness matrix. The standard finite element formulae for nonlinear complete analysis are derived. The acquired element stiffness matrix can be partitioned into three submatrices reflecting three different nonlinear effects: material nonlinearity, coupling of material nonlinearity and large element displacement, and secondary moment of axial force, respectively. The results agree well with the experimental ones.