Greenhouse and laboratory studies were conducted to investigate allelopathic potential of some weed species on alfalfa(Medicago sativa L.) germination and seedling growth. In the comparison between top(leaves+stems) and root extracts, top extract exhibited greater allelopathic effects on alfalfa germination than that of root. The various weed species extract differently responded to alfalfa test species, WL-320, in terms of allelopathic effect. Top and root aqueous extracts of lambsquarter(Chenopodium album L.), giant foxtail(Setaria faberii Herrm.), redroot pigweed (Amaranthus retroflexus L.), velvetleaf(Abutilon theophrasti Medic.), crabgrass(Digitaria sanguinalis L.), canada thistle(Cirsium arvense L.) and prostrate knotweed(Polygonium aviculare L.) significantly inhibited germination, seedling length, weight, vigor, and rate of germination of alfalfa. The regression slopes of various top extracts showed that velvetleaf(b=3.69) extracts were the most inhibitory, while large crabgrass(b=2.39) extracts had the least allelopathic effect on alfalfa germination. Germination, seedling length and weight of alfalfa were inversely proportional to the concentration of dried velvetleaf extracts. Also, more of the toxic effects were observed from the dried extracts compared to the fresh extracts. Residue of velvetleaf inhibited significantly alfalfa emergence and survival percentage compared to the control. The emergence and survival percentage of alfalfa were 44%, 57% at 1.0% residue treatment, respectively. When weed residues were mixed with silica sand with incubation time, velvetleaf residue most inhibited alfalfa growth. The degree of inhibition increased as incubation time increased. An incubation for 72h caused the greatest inhibition of alfalfa growth. These results demonstrate the different allelopathic activity of weed species extracts on alfalfa and suggest that weed may affect alfalfa growth and development through the inhibitory effects of allelochemicals present in weed tissue.