It has been known that plastic deformation of steel members causes thermal stress attenuation in a steel frame at fire. Although existing design guidelines consider this thermal attenuation behavior, design equations for estimating the ultimate strength of the frame do not directly consider the thermal stresses in the members. This is introduced based on a fact that the thermal stresses do not affect the ultimate strength due to the thermal stress attenuation. It should be noted, however, that this condition is only possible when the members have large plastic deformation capacity. To have better understanding of this thermal attenuation behavior, an investigation has been conducted in this paper on the structural fire resistance of a statically indeterminate steel beam at elevated temperature. Since the plastic deformation capacity is related in part to the strain hardening of steel, a relationship between the strain hardening and thermal stress attenuation is emphasized. Based on both theoretical and numerical analyses of the steel beam subjected to the thermal stresses, this paper clarifies that the strain hardening of steel plays an important role in attenuating the thermal stress; therefore the strain hardening contributes directly to improving the structural fire resistance.