Mixed structures composed of R/C (Reinforced Concrete) columns and steel beams can accommodate larger building spaces, whereas the R/C columns may be subjected to severe thermal stresses due to the thermal elongation of long span steel beams in a fire. In previous paper, results of load-bearing fire tests of sub-frames of the mixed structure indicated that the fire resistance of R/C columns is reduced by covered concrete spalling and forced horizontal displacement at the top of the columns due to thermal elongation of steel beam. In this paper, results of thermal stress analysis of these tests are presented and the influence of the concrete spalling and the action from the beam on the fire resistance of R/C columns is discussed.