In order to investigate the allelopathic effect of walnut leaf extract on germination characteristics of wheat, onion and Lactuca, a research was conducted at faculty of agriculture, Uremia University in 2007. In this experiment, the effects of different concentrations of walnut water leaf extract including control (distilled water), 2.5, 5 and 10% on germination characteristics on three plants: wheat, onion and Lactuca separately was studied in completely randomized design (CRD) with three replications. Increasing concentration of leaf extract lead to continuously decrease in germination percentage, germination rate (GR), time to reach 50% germination (T50), radical and plumule length and seedling fresh and dry weights. The most inhibition effect on germination percentage was observed in Lactuca. The most and the least decrease in germination rate, concerned to Lactuca and onion of 59.44 and 22.23 percentage, respectively. Also, the most inhibition effect on time to reach 50% germination was obtained in wheat. The most inhibition effect on steam and root length in wheat plant was resulted in 88.47 and 80.11 percentage in 10% concentration than the control, respectively. Greatest decrease in fresh and dry weight concerned to wheat because of its high sensitivity than other plants. Furthermore, onion showed the least sensitivity to juglan allelopathic substances.