Abstract: In this paper, deriving directly from the stress-strain relationsfor concrete and steel, the relation among moment, normal force and curvature (M-N-φ) on PC (Prestressed Concrete) sections is determined by the method presented. The methods of determing the M-N-φ relation for the beam section and the column section of a frame are unified. The displacement mode and stiffness matrix for element with variable stiffnessare proposed. The nonlinear analysis for PPC (Partially Prestres sed Concrete) frames is implemented. The obtained results by the non-linearanalysis are in good agreement with the results of the four PPC frame tests conducted by the author.