Abstract: The thermo-sensitive period and the critical temperature of fertility transition of C49S, a principal thermo-photo-sensitive genic male sterile line in two-line hybrid wheat, was studied in the growth chambers for controlling temperature and photoperiod.The seeds were sown on different time for some years. The results showed that the thermo-sensitive period in fertility expression of C49S was from PMC formation stage to mature pollen stage, and there were two most sensitive stages to temperature on fertility expression. One was the PMC meiosis stage, and the other was the middle microspore stage. The critical temperatures evoking a complete male sterility were the mean minimum temperature at PMC meiosis stage (Tmin₁),the mean temperature at microspore stage (T₂) and the mean minimum temperature at microspore stage (Tmin₂) lower than8.5℃,13.5℃ and 10.5℃, respectively.The critical temperatures keeping a nearly normal male fertility Tmin₁and T₂ and Tmin₂ were higher than11.5℃,15.0℃ and 12.5℃,respectively. The value as well as the conditions and the risks of thermo-photo-sensitive genic male sterile line of wheat applied to hybrid wheat were evaluated in this paper.