Abstract: 1.The hybrid rices "Ai-yu No. 2" and "Nan-yu No. 2" are weakly photosensi- tive types. Under 10 hours of day-length their heading is accelerated by 18–32 days than that under natural day-length. The corresponing male-sterile and maintainer lines are insensitive or very weakly sensitive to short day-length. Their range of acceleration of heading is only 0–3 days. 2. The photoperiodic sensitivity of hybrid rice is determined mainly by their paternal line (the restorer line) to day-length. This should be taken into consideration in selecting new restorer lines. 3. The vegetative period of "Nan-yu No. 2" under the short day-length is shortened by 32 days, while its paternal and maternal lines by 10 days and 1 day respectively. Therefore, inheritance of photoperiodic sensitivity in this hybrid rice is transgressive. 4. Hybrid rices "Ai-yu No. 2" and "Nan-yu No. 2" display obvious heterosis in comparison with the restorer line IR24 and the control strain "Chen-chuai". 5. Under the natural day-length of Wuchang both hybrid rices have 17 leaves on the main stem while IR24 has 18. 6. Differentiation of young panicle in hybrid rice occurs at the age of 13–13.5 leaves. Panicle differentiation begins in the male-sterile lines later than in the maintainer lines by 2–5 days. The date of heading of the male-strile lines is later by 2–3 days. For propagation of male-sterile line or hybrid seed production, it is important to arrange carefully the sowing and transplanting times to make the flowering periods meet, since this difference of heading period between the male-sterile line and the maintainer line is not affected by photoperiodic conditions.