A numerical model, in the form of a computer program, is presented for tracing the fire behavior of reinforced concrete (RC) beams over the entire range of loading from pre-fire conditions to collapse under fire. The three stages associated with the analysis of fire resistance; namely, establishing the fire temperature—time development, calculating the heat transfer through the structure from the fire, and the structural analysis are explained. The model, which accounts for nonlinear material properties at elevated temperatures, is capable of predicting the fire resistance of RC beams under realistic fire scenarios, load levels, and failure criteria. The validity of the numerical model is established by comparing the predictions from the computer program with results from full-scale fire resistance tests. Through the results of numerical study, it is shown that the type of failure criterion, load level, and fire scenario have significant influence on fire resistance of RC beams. The computer program can be used to undertake performance-based fire safety design of RC beams for any value of the significant parameters, such as fire exposure, concrete cover thickness, section dimensions, concrete strength, concrete type, and load intensity.