Heat tolerance at the heading-flowering stages for 37 early-season rice varieties, which were released in Jiangxi province, were evaluated under natural and high temperature conditions. Heat tolerance of rice varieties was evaluated using seed setting rate (SSR) and its reduced rate under high temperature. The result showed that the F values of variance analysis for SSR under nature and high temperature conditions and its reduced rate in hybrid rice combinations were 1.7-1.8 (F0.05=1.9), which were close to significant level, it means that there are definite difference of heat tolerances among hybrid rice combinations. The F values of variance analysis for SSR under nature and high temperature and its reduced rate in conventionality rice varieties were 0.8-0.9 (F0.05=3.8),which were no significant, it means that there were a very smaller difference of heat tolerances in conventionality rice varieties. The average of SSR for hybrid rice combinations in 2013 was reduced than 2012 for 8.9%, while the average of SSR for conventionality rice was only reduced 1.72% than 2012, when the average of maximum temperature (41.3 ℃) under high temperature stress was more than 2012 for 0.9 ℃, the reduced rate of SSR for conventionality rice was obviously lower than hybrid rice combinations; so the heat tolerance of conventionality rice were stronger than hybrid rice combinations. In natural condition, the average maximum air temperature of 2013 was 36.8 ℃, higher than 2012 for 1.3℃, but the average SSR of hybrid rice combinations and conventionality rice were more than 2012 for 5.6% and 15.0%, respectively. This results were caused by the relative humidity (81.4%) of 2013 in nature condition, which was higher than 2012 for 14.4%. It indicated that the high tolerance of rice varieties will become stronger with the increase of relative humidity, when the air temperature was higher than suitable growth temperature. Four hybrid rice combinations of Wufengyou 623, Lingliangyou 611, Zhuliangyou 819, Zhuliangyou 312, and four conventionality rices of Xiangzaoxian 7, Jiayu 948, Zhongzao 35, Zhongzao 25 were showed more SSR and smaller reduced rate under high temperature stress, which could be used in rice production and breeding with heat tolerance.