This paper presents experimental studies on the performance of planar moment steel frames with one column heated. The novelty of these tests is that the influences of key stages in the response of the heated column under gravity loading, including postbuckling and cooling phase after fire, on the progressive collapse resistance of steel frames were studied. Gas temperatures of the furnace, structural member temperatures, displacements, and strains in certain locations of the test frames were monitored during the tests. The test results show that steel columns with strong constraints from steel beams in a frame may fail in a quasi-static manner under fire, whereas those with weak constraints or large load ratios may fail with dynamic effects. In addition, the final equilibrium vertical displacement at the top of the column after sudden buckling can be even larger than the final displacement associated with gradual column removal. Moreover, in the cooling phase after fire further damage may occur to other members of the frame because of the thermal contraction of the heated column.