Abstract Background & Objectives: Germination is one of the essential period of plants growth that can be affected harmfully by chemical agents released by other palnts.The allelopathic effects of Redroot pigweed ( Amaranthus retroflexus L.), Lambsquarters (Chenopodium album L.), Bermuda grass (Cynodon dactylon L.), Nut grass (Cyperus rotondus L.), Black nightshade (Solanum nigrum L.) and Datura (Datura stramonium L.) were investigated on germination characteristics of Marigold (Calendula officinalis L.) seeds in a growth chamber experiment. Material and Methods: Experimental treatments were extracted by distillation from dried leaves of six species of weeds that applied by four concentrations included: zero (distilled water), 2.5, 5.0 and 7.5 volume percent (w / v) was applied on the seeds of Marigold plants. Experimental design was factorial experiment in a completely randomized design with four replications. Results: The aqueous extracts applied to seed, significantly inhibited seed germination, germination rate, radicle and hypocotyl length of marigold with increasing of extract concentration. Bermuda grass extract showed the most inhibition effect on seed germination of Marigold and decreased it by 57/7%. The inhibition effect of Datura, Nut grass and Black nightshade extracts was less than Bermuda grass. Datura, Bermuda grass and Nut grass extract showed the most negative effect on germination rate of marigold and decreased it 68%. All weeds extracts significantly decreased the length of marigold radicle and hypocotyl. But Lambs quarters and Nut grass extracts showed the most inhibition effects. Conclusion:These results suggest that extracts from shoot of weeds had potentially allelopathic effect. However the activity differed depending on weed species and extract concentration.