Abstract: Different sowing times and on-farm observation are used to investigate field climatic factors coinstantaneous with harmful high temperatures and their effects on hybrid rice fertility at anthesis for the year 2005. The results show that daytime temperature and light density in high and normal temperature periods are significantly different in the hours of 10:00 ～14:00, but much similar in the hours of 8:00～10:00 and 16:00 ~ 18:00. Though daytime relative humidity is lower in high temperature than in normal temperature periods, the difference is insignificant. High temperature-driven climatic factors induce significant poor seed-setting, decreasing on the average of 8.0% for three cultivars. Under high temperature-driven conditions, the percent of pollen vigor and pollen number per stigma decrease sharply, up to 6.0% and 8.4 pollens respectively, when compared with normal temperature conditions. Seed-setting rate, however, shows no significant difference among three cultivars. Comparison to “Shanyou63”, “Jinyou448” and “Liangyoupeijiu” show a relatively large difference in seed-setting rate, percent pollen vigor and pollination per stigma under the two climatic conditions. This implies that “Shanyou63” is more tolerant to high temperature-driven climatic conditions than the other two hybrids.