The configuration of a steel beam in a concrete/steel composite floor in fire gives rise to a non-uniform temperature profile across the depth of the cross-section. This temperature profile affects the deflection of the steel beam in two ways: thermal bowing due to non-uniform thermal strains and beam deflection due to imposed loads. The beam deflection becomes larger as the elastic properties of the steel beam deteriorate when the temperature is rising. The deflection increases rapidly when the cross-section of the steel beam starts to yield. This paper presents a method for the calculation of the total deflection of a steel beam in a steel/concrete composite floor in fire when the beam is loaded beyond its elastic limit. In this study, the steel beam is assumed to support the concrete floor slab simply at its ends without composite actions.