In order to study the effects of different cover crops in suppressing weed growth, an experiment based on the randomized complete block designs was carried out in the agricultural research station of Mohaghegh Ardabili University in 2012-2013 growing season with four replications. Treatments included fall rye (Secale cereale), spring vetch (Vicia villosa), and berseem clover (Trifolium alexandrium) along with the control treatment. Some traits of weeds and cover crops were measured 60 days after cropping (cover crops flowering) and 90 days after cropping (one month after cover crops were clipped from the ground level). Results of the analysis of variance showed that in two stages of sampling, plant density, crop height, fresh weight and dry weight of cover crops had significant differences. Fall rye, spring vetch and berseem clover decreased the density of common lambsquarter (Chenopodium album) compared with control about 69, 81 and 88 percent and dry weight of this weed in the second sampling about 73, 96 and 93 percent, respectively. Evaluation of the phonology weeds during the first sampling showed weeds were in vegetative phase along with the winter rye and spring vetch but were in the flowering phase accompanied by berseem clover, like control. In the second sampling stage, all three cover crops suppressed the growth of the weeds and kept them in the vegetative stage. For the bindweed (Convolvulus arvensis), percentage of decrease in density compared with control was about 77, 55 and 9 percent and dry weight loss was measured to be 78, 61 and 33 percent, respectively. Effect of the three cover crops on the control of the bindweed was lower than that of the Common Lambs quarter. This higher competitive potential against cover crops probably is due to the reserving organs. Cover crops of fall rye and spring vetch were the most suitable crops to weed control in the first stage, after control and Berseem clover was distinguished as the non-suitable one.