This paper presents the results of nonlinear finite element analysis of reinforced concrete (RC) frame structures, including the complete behavior of these systems from zero load to the ultimate load. Presented also are the details and results of a corroborative test program involving a large-scale frame model. The effects of the finite element size and tension-stiffening on the nonlinear responses of three RC frames are investigated. In addition, the program is used to carry out a "plastic" analysis of the frame to define the mechanism of failure, the plastic hinge rotations, and the yielding and the plastic hinge lengths. The capability and accuracy of the nonlinear finite element analysis program in predicting the nonlinear response of RC frame structures is verified, along with a comparison between the analytical and the corresponding experimental results. The different behavioral aspects including cracking, yielding and ultimate loads, load-displacement and load-strain characteristics for concrete and reinforcing steel and plastic hinge deformations are studied. The analytical and experimental results indicate that the computed response of RC frame structures is strongly influenced by the finite element size. The finer meshes give lower values of the ultimate load and vice versa for the coarser meshes. With an increase in the number of elements, the structure is slightly more flexible than for the case for the coarse mesh idealization, and the frame tends to be less ductile. An empirical equation has been proposed to eliminate this drawback. The calculated plastic hinge rotations show good agreement with the experimental results. For example, the maximum deviation between the analytical and the experimental values of the plastic hinge rotations is approximately 13%, while the minimum deviation is 4%.