Abstract: Eucalyptus citriodor Hook.is an important tree spedes of plantation in South China.The allelo pathic effects of aqucous extract of bark,leaf, root exudates and leaf volatiles on the germination and seedling growth of radish were studled.The results indicated that there are allelochemicals existed in aqueous extract of bark,leaf,root exudates and volatiles.Different sensitivity to inhibitions were existed in 9 acceptor plants.The inhibitor from aqueous extracts which has reversible effect to other plants were not easy to stay insoi.Volatiles inhibitions were easy accumulated in soil,its'inhibitory effects is inreversible.The inhibitory effects were significantly negatively correlated with the monthly rainfall in a year.