Abstract: In order to understand the resistance of a steel beam of a structural system in fire, the experimental behavior of a two-span consistent beam of a three-span and three-storey steel frame building subjected to fire is studied with the help of the self-designed furnace and detecting devices. There are only steel beams subjected to fire during the whole test. The temperature of furnace gas, steel beams and concrete floor section temperature at four equal points along the beams, the temperature of steel beam joints located outside the furnaces and the vertical deformations at four equal points along the beams were measured. The results showed that the temperature curves of exposed steel beams were consistent with the furnace gas curve, the temperature rise of concrete was slower than that of steel beams, the temperature of the beams decreased rapidly after the heating, but concrete temperature continued to rise until the two curves were in consistency. In addition, there was a big temperature gradient across the concrete section throughout the experiment. The steel beams without fire protection in the building performed well; therefore, the fire protection of it can be reduced. Fire resistant design of components at different positions should be treated differently, for the performance of different components was varied in the fire due to the constraints of adjacent components. While steel columns generated an axial expansion during the test though they are not exposed to fire.