Abstract: The influence of osmotic effects of aqueous plant extract and mannitol solutions on the results of allelopathy bioassay was investigated for three crops including radish，cucumber and Chinese cabbage ．The results show that with the elevation of osmotic potential，the root length and seedling height of radish and cucumber decrease gradually，while the influence on the growth of Chinese cabbage is not obvious．The results differ when data from deionised water and mannitol solutions with the same osmotic potential are used respectively during the inves tigation of three main dominant species such as Cryptocarya chinensis，Castanopsis chinesis and Castanopsis fissa．It is found that allelopathic effects are substantially overes timated when osmotic effects are not taken into account It is concluded that bioassa ysusing aqueous plant extract are more realistic when osmotically adjusted control values are used．