Aqueous extract of leaves, bark and seeds of Meliaazedarach.L and Thujaorientalis L. were assayed at 1, 5 and 10g/L concentration with different time period to check their effect on seed germination, fresh and dry weight and seedling growth of Pennesitiumamericanum L. Result revealed that aqueous extracts of both plants at all concentration and time period had significantly inhibited seed germination ofP.ammericanum L. when compared with control. The inhibitory effect increases with increasing concentration of extracts and time period. The bark extract of all concentration of T.orientalis in 24hr and 48hrs show stimulation in seedling growth while other parts and extracts of all parts of M.azedarachin all concentration and time period show inhibition in all parameters. The order of inhibition when compared different parts of M.azedarch was seeds >bark > leaves while that of T.orientalis was leaves > bark >seeds. Hence it is concluded that aqueous extract of these plant contain water soluble allelochemicals which inhibit the seed germination and seedling growth of P. americanum L. It is suggested that these chemicals may be used as herbicides.