This paper describes an experimental and numerical study at both ambient and elevated temperatures on the behaviour of full-scale composite floor cellular steel beams. A total of four specimens, comprising two different steel geometries and loading conditions were tested under monotonic loading and at elevated temperatures. All beams were designed for a full shear connection between the steel beam and the concrete flange using headed shear studs. The beams were designed to fail by web-post buckling, which was observed in all the tests. Failure temperature observed in the fire tests indicated that failure by web-post buckling of cellular beams in fire cannot simply be estimated by applying temperature dependent reduction factors on stiffness, as given in codes. A finite element model is then established with both material and geometrical non-linearity using shell elements to compare against the experimental results. The comparison between the finite element prediction and actual test results are quite good in terms of failure modes, load deflection behaviour and ultimate loads. (C) 2007 Elsevier Ltd. All rights reserved.