Abstract: The agronomic traits including plant height, panicle length, seed set, 1000-grain weight, panicles per plant, grains per panicle and theoretic yield per plant of 5 thermo-tolerant japonica rice cultivars ("Dongdao 2", "Dongnong 03-33", "Dongnong 415-1", "Hejiang 195" and "Jijing 3") from Northeast China were evaluated and the relationships between nature air temperature and seed set were analyzed by use of interval sowing populations (Mar. 20, Mar. 30, Apr. 9, Apr. 19, Apr. 29, May 9, May 19 and May 29 in 2011) at Changsha, and then the thermo-tolerance at the heading stage of them was summarized. Results showed that when sowing date was postponed, the days to flower became short gradually. There were wide variations in panicles per plant, grains per panicle and theoretic yield per plant. Meanwhile the variations in plant height, panicle length, seed set and 1000-grain weight were small among different sowing dates. A negative correlation was noted between daily mean and maximum temperature and seed set. Even though the seed set of "Hejiang 195" was affected least by air temperature, the seed set of "Dongdao 2", "Dongnong 03-33" and "Dongnong 415-1" correlated with air temperature significant at the 5% level, while that of "Jijing 3" significant at the 1% level. Within a certain range of temperature, the regression curve for seed set and air temperature was cone-shaped. Furthermore, the effects of daily maximum and mean temperatures on seed set of rice were consistent. Among the interval sowings that coincided with high temperatures at the heading stage, the seed set varied in the range of 64.9%~75.1%, beside that of "Dongnong 03-33" sowed on May 29, and "Jijing 3" sowed on May 19, whose seed sets were 58.7% and 52.5%, respectively. These cultivars exhibited more reliable thermo-tolerance among the tested rice cultivars. Based on the analysis of agronomic characters of interval sowing and the relationship between nature air temperature and seed set, it was recommended to plant "Dongdao 2" and "Hejiang 195" as early season rice in Hunan Province.