The performance of steel reinforced concrete (RC) beams with a box cross section exposed to fire is studied. The cross-section is divided into an appropriate number of segments so that non-uniform temperature profiles and variations of constitutive relationships across the section can be represented accurately. The temperature distribution in the cross-section is calculated by the finite difference method. The nominal moment capacity of RC beam is obtained using equilibrium of forces in the segments of beam. Advantage of circulating cold water and cover concrete on the nominal moment capacity under fire is examined. Results obtained by the prepared computer program were found to predict the fire resistance and performance of RC box beams well