During lasing process, the active medium is not cooled actively in heat capacity laser, so the temperature of the active medium increases sharply. As a result of the decreasing stimulated emission cross section with increasing temperature, the laser gain declines obviously. Based on the relation between the temperature and the stimulated emission cross section of Nd∶phosphate and 1% doping Nd∶YAG, the changes of power output and pump threshold in heat capacity lasers are calculated. The results show that the power output declines obviously and the pump threshold rises sharply with increasing temperature in heat capacity lasers because of thermally induced change of stimulated emission cross section.