Abstarct: Allelopathy is a natural and eco-friendly technique that refers to chemical contrast between plants. It is a unique method to control weeds and consequently to increase yield of crops. In this regard, in order to evaluate the effect of wheat and barley straw on weeds biomass and sesame yield, an experiment was conducted at the research field of Shahrood University of Technology as randomized complete block design with four replications in 2014. Treatments includes weeding, no weeding, Trifluralin (EC 48%), foliar application of alcoholic extract of wheat straw at concentration of 50 and 100 percent, 2, 4 and 8 tons of wheat straw per hectare, and foliar application of alcoholic extract of barely straw at concentration of 50 and 100 percent. Results showed that weeding treatment had the greatest positive effect on the most of traits. Seed and biological yield of sesame increased 64 and 48 times in weeding than in no weeding treatment, respectively. Weeding treatment was the best treatment in control of weeds, but due to the high density of barnyard grass, other treatments of experiment had no significant effects on density and dry weight of narrow leaf weeds in comparison to no weeding treatment. Broadleaf weed density in 4 tons of wheat straw per hectare treatment was significantly more than no weeding treatment. Other treatments except weeding had no effect on density of the broadleaf weeds in compared with no weeding treatment. Also dry weight of broadleaf weeds significantly decreased by foliar application of alcoholic extract of wheat straw, foliar application of alcoholic extract of barely straw at concentration of 50 percent, 2 and 8 tons of wheat straw per hectare treatments in compared with no weeding treatment. Generally, results showed that in the high density of weeds, weeding was the most effective method for weed control. Also using wheat straw mulch or foliar extract of wheat and barley will not be able to reduce density and weed biomass especially narrow leaf weeds in high densities.