The heat release rate is the key factor that determines fire hazards. In order to find out the characters of heat release rate and their relation with other parameters. The authors introduce the oxygen consumption principle which cone calorimeter applies, and then expound the calculation of heat release rate in detail. The heat release rate of several kinds of representative materials has been studied by means of cone calorimeter in the different irradiances. Test results shows that HRR and TFI^-1/2 vary linearly with the irradiances.