Abstract: The effects of exogenous gibberellin（GA3）and paclobutrazol（PP333）on tuber expansion and bulbil formation were studied with big tuber variety‘Guihuai 5’and little tuber variety‘Guihuai 16’as plant materials. The results showed that yam tuber yields were increased significantly by application of exogenous GA3 and its optimum concentration was 200 mg ? L-1，while tuber yields were decreased significantly by application of exogenous PP333 at 200 mg ? L-1. The tuber development of Chinese yam could be divided into three periods，which are tuber formation period，tuber expansion period，and tuber maturity period. Compared with the control，exogenous GA3 decreased significantly total tuber weight perplant as well as single tuber weight on 150 days after field planting during tuber expansion period，and　increased significantly tuber number per plant as well as total tuber weight per plant during tuber maturity period. In the meanwhile，exogenous GA3 increased significantly bulbil length and the weight of single bulbil，but decreased the total weight of bulbil per plant compared to the control. Compared to GA3 treatment，exogenous PP333 decreased significantly total tuber weight per plant and single tuber weight，increased bulbil width and total weight of bulbil per plant during tuber expansion period，but decreased significantly tuber number per plant and single tuber weight during tuber maturity period.

Key words: Chinese yam, gibberellin, paclobutrazol, tuber；bulbil