The allelopathic potential of Mikania micrantha H.B.K. to affect the seed germination and seedling growth of Coix lacryma-jobi L. was investigated. Water-soluble allelopathic substances were found in the water extracts of M. micrantha. The effect of the water extracts on the seed germination and seedling growth of C. lacryma-jobi was concentration-dependent. The water extracts from the different plant parts (leaf, stem, and root) of M. micrantha differed in their effect on the germination and seedling growth of C. lacryma-jobi, with the effect of the leaf extract being the least inhibitory. The malondialdehyde (MDA) content in the C. lacryma-jobi seedlings increased by 64%, 45%, and 52% of the control with increasing concentrations of the extracts of the root, stem, and leaf (80, 400, and 400 $g;L^{-1}$, respectively). The extract from the M. micrantha roots significantly increased the catalase (CAT) activity of the C. lacryma-jobi seedlings (48% and 54% of the control at the concentrations of 20 $g;L^{-1}$ and 80 $g;L^{-1}$, respectively). The extracts from the leaves and stems at low concentrations increased the CAT activity, but at high concentrations, the extracts decreased the CAT activity. The extracts from the roots, stems, and leaves at concentrations of 80, 400, and 400 $g;L^{-1}$ also significantly decreased the peroxidase (POD) activity of the C. lacryma-jobi seedlings to 27%, 52%, and 34% of the control, respectively. These results indicate that the water extracts of M. micrantha could inhibit the seed germination and seedling growth of C. lacryma-jobi through the regulation of anti-oxidase activity, such as POD and CAT in the cells. The growth inhibition of the C. lacryma-jobi seedlings is probably related to injury after oxidization of the cell membranes with the increase of MDA content.