In order to analyze the behavior of building structures in a real fire, a numerical method is proposed, which employs fire zone model to simulate the fire growing and the temperature distributions in a firing room. Two computer programs are developed to predict the thermomechanical response of the structures, one for thermal response analysis, and another for structural analysis. The structural analysis program contains a fiber beam element and a layered shell element. These elements, validated against data from experimental tests, can be used to predict the response of beam/column and slabs in fire conditions. Based on these programs, a system for analyzing the behavior of structures under fire is developed on MSC/PATRAN platform. The behavior of building structures under fire can be simulated with this system and the results can be used as the reference for the fire safety analysis and assessment of general building structures under fire.