Abstract: Eucalytus exserta and E. urophylla are two important tree species of man-made forests in South China. Leaf volatiles of them had significant inhibitions on seedling growth of Raphanus sativa, Lactuca sativa,Leucaena leucocephala and Acaia mangium. In comparsion of allelopathic effects of the three fractions from the distillation of fresh leaves, the declining order was: water soluble unvolatile chemicals > volatile oil > water soluble volatile chemicals. Volatile oils of the two Eucalytus could completely inhibite the seedling growth of A. mangium when volatile oil concentrations are 0.2 milliliter per culture dish. Allelopathic effects of volatile oil decompositing in soil would decline very quickly within 15 days. After that, it would maintain a certain level with significant inhibition. The aqueous extracts of fresh leaves of the two plants showed inhibitory effects on seedling growth of radish and luttuce. The aqueous extracts of E. exserta also inhibited the seedling growthg of A. mangium. The aqueous extracts of E. urophylla inhibited the seedling growth of L. leucocephala.