Abstract: The aqueous extract of Melilotus officinalis is known to inhibit weed growth. To study in detail the allelopathic inhibition of grass growth and elucidate the allelopathic mechanism, an aqueous extract of M. officinalis was applied to seeds and seedlings of Lolium multiflorum and Sorghum sudanense and its effects on their growth were evaluated by measuring several important physiological and biochemical indexes. The aqueous extract of M. officinalis significantly (P<0.05) reduced the germination rate, root length, root dry weight, stem length, stem and leaf dry weight of both test species, compared with their respective controls. The aqueous extract of M. officinalis significantly reduced the chlorophyll content and increased the starch and soluble protein contents (P<0.05) of both test species. Treatment with the aqueous extract of M. officinalis significantly (P<0.05) increased the soluble sugars content in S. sudanense and significantly (P<0.05) reduced the soluble sugars content in L. multiflorum. The catalase activity in S. sudanense treated with the M. officinalis extract differed significantly (P<0.05) from that of its control, but returned to control levels after 8 days. The peroxidase activity in S. sudanense treated with the M. officinalis aqueous extract first decreased and then increased.