Abstract: Introducing the concept of unbalanced forces between internal forces and external loads, an approach for analysis of nonlinear response of steel frames subjected to fire is developed.The actual temperature elevation of structures in real fires can be followed by this approach. Both the temperature-dependent material nonlinear ity and geometrical nonlinearity are taken into account. The non-uniform profile of temperature across the cross section of structural members is also considered.The fire-resistant test on three models of steel frames carried out at Tongji University in 1996 is presented.The validity and effectiveness of the approach is verified by good prediction to the nonliear behavior of steel frames tested in fire.