Abstract：Watermelon〔Citrullus lanatus（Thunb.）Matsum. et Nakai〕allelopathy was studied by bioassay method，and allelochemical in root，stem-leaf and root exudates were detected by GC-MS．The results showed that watermelon seed germination and seedling growth were promoted at 2.5 g?L-1 concentration of the root and stem-leaf aqueous extracts，while they were restrained under condition of their 5.0 g?L-1 concentration．The inhibiting effect became stronger with the increasing of concentrations．The inhibiting effect of stem-leaf was more than that of root at the same concentration．The root exudates promoted the seed germination and seedling growth at 2.5 μL?L-1．It began to restrain the seed germination and seedling growth at the point of 5.0 μL?L-1．32 chemicals were detected from root，stem-leaf and root exudates，of which 14 were esters chemicals．There were 14 in root，19 in stem-leaf，and 22 in root exudates，some of these chemicals which were previously reported were allelochemicals．The results revealed that there were obvious differences in different tissues of watermelon，so allelopathy was different.
