In order to weeds control in onion (Allium cepa), the research was conducted during cropping year of 2013 at Shabestar county, Iran in form of factorial based on randomized complete block design with three replications. The treatments were included: factor of three onion cultivars (Azarshahr red onion, Zargan and Swaet spanish) and factor of weeds control at five levels (control, mechanical weeding, Oxyfluorfen herbicide with amount of 480 g/ha-1 effective material, Oxadiazon herbicide with amount of 360 g/ha-1 effective material, composite of Ioxynil and Sethoxydim herbicides with amounts of 675 g/ha-1 and 250 g/ha-1 effective materials, respectively). Based on the results of variance analysis, mechanical method had the highest percentage of weeds control and caused to significant reduction in weeds dry weight and augmentation in yield was 75%. Among herbicides, Oxyfluorfen after weeding could have the highest effect on increment of yield (62%). Maximum yield was obtained in cultivar of swaet spanish and hand weeding treatment.