Abstract: Experimental research and nonlinear theoretical analysis of the ultimate strength of steel frames are presented in this paper. 4 steel frames in which 2 are two-story onebay frame and other 2 aretwo-story two-bay frame were tested. A suitable nonlinear analysis method, which is based on nonlinear continuum mechanics, stress-resultant yield surface and displacement increment Newton-Raphson subincrementation scheme, is successfully applied to the large deformation elasto-plastic analysis of the ultimate strength of steel frames.The comparison of the theoretical predictions with the experimental results tested by the author verifies that the theoretical method derived in this paper has very high precision and efficiency.