The design principles for high strength steel (HSS) structures exposed to fire are under development. In this paper, the response of HSS structures in fire is studied through numerical simulation of a beam and a two-bay frame. Geometrical imperfections and residual stresses are introduced into the structural models. Deformation limit criteria are used to compare the critical temperatures of the structures made of HSS and mild steel. The comparisons show that HSS structures have higher strength reserve than mild steel structures. Using the mechanical properties at elevated temperatures from literature sources, it is observed that the deflection behaviour of the studied structures depends on the ratio of strength to elastic modulus. The deflection of the studied beam is sensitive to yield strength reduction factors as the beam fails by plastic hinge mechanism. Whereas, the deflection of the HSS frame is sensitive to the reduction factors of the elastic modulus as the frame fails by inelastic instability. The above-mentioned observations on the studied structures are made using a three-stage mechanism which is developed for interpreting the deformation response.