Filter paper method using petri dishes was applied to determine the self-allelopathy of Ageratum conyzoides L. and Bidens pilosa L. Effects of aquatic extracts from roots, stems and leaves of each plant were tested under concentrations of 0, 0.10, 0.25 and 0.50 g/mL on seedling root length, height and fresh weight of their own plant. The result showed both plants exhibited self-allelopathy: aqueous extracts showed obvious inhibition on seedling root length, the inhibition effects increased with increasing concentration of the aqueous extracts；showed double effects on the height of the seedling, which promoted seedling height in low concentration but inhibited in high concentration；and showed marked inhibition on fresh weight under higher concentrations.