Abstract: The behaviors of three simply_supported compos ite s labs and four continuous compos ite s labs subjected to fire with constant load were experimented. The distortion,destruction form and internal force redistribution of the slabs were investigated. It can be shown from the experiments that the fire resistance capability of simply_s upported s labs is bad,while that of continuous slabs is good due to the pos itive effect of negative tensile reinforcement. It can also be shown that both in temperature_rise period and temperature_descent period,there is sharp redistribution of internal force in continuous slabs,and the different fire_s uffering conditions have great effect on the behaviors of continuous slabs. At the s ame time,the time, positions and orders of the plastic hingesc appearance also have great effect on the response of the continuous slabs. At last, a s imple method is proposed to analyze the variety of support reactions of continuous s labs during the temperature_rise period,and the calculated results fit well with the experimental results.