In this experiment, super hybrid rice Zhongzheyou 1 and Guodao 6 were used to study the effect of different age（12 d, 17 d, 22 d, 27 d） on seedling quality and yield under the mechanized transplanting system. The results showed that the plant height and dry weight were increased with the increase of seedling age, however, at the late stage of rice seedlings, the leaf grew slowly and root-shoot ratio decreased. There was no significant difference on no-seedling rate between the different seedling age; the short-age seedlings turned green faster after transplanting in field and there were more effective panicles compared with long-age seedlings. The grain yield of 17 d seedling age treatment was the highest because of more panicles, higher seed setting rate and 1 000-grain weight. With the increase of transplanting seedling age, the growth period of rice will be postponed. In conclusion, the appropriate seedling age for single season rice was 15~20 days in the middle and lower reaches of the Yangtze River, because of better seedling quality and higher grain yield, and was suitable for machine transplanting.