Abstract: Effects of six treatments of different seedling ages on yield and agronomic traits of single-cropping Indica-Japonica rice “Yongyou 12” were studied in this study. The results showed that, within 15-40 d seedling age, the rice yields in the treatments of 20 d seedling age and 30 d seedling age were significantly higher than the treatment of 40 d seedling age, but were no significant different to other seedling ages. It was revealed that, the seedling ages within 15-35 d were suitable ones and 20-30 d were most adequate ones.