Onion is one of the most important vegetable in India. However, yields are generally low due to weeds problem which reduce onion yields. In order to evaluate various  herbicides for weed control in onion, an experiment was conducted at Agricultural Research Institute, Patna under Bihar Agricultural  University, Sabour, Bhagalpur during two consecutive rabi Season 2016-17 and 2017-2018.The experiment comprised 3 herbicides and were applied alone , their combinations ,hand weeded plot and a weedy check was also included. Thus the eight different treatments were: weed free throughout the all crop period, two hand weeding at 20 and 40 days after transplanting, glyphosate at the rate 1kg  active ingredient /ha 15 days before transplanting, pendimethalin at the rate 1 kg active ingredient/ha as pre emergence i.e., 3 days after transplanting, oxyfluorfen  at the rate  250 g  active ingredient /ha as post emergence i.e. 30 days after transplanting, pendimethalin followed by oxyfluorfen, glyphosate followed by pedimethalin followed by oxyfluorfen and the control.  Statistical analysis of the data showed that the hand weeded treatment had the lowest  total dry weight of weeds (10.3g) and total fresh weight of weeds (29.4g) followed by combined application of glyphosate, pendimethalin and oxyfluorfen treated plot (12.7g) and (24.5g) as compared to the weedy check (45.8g) and (159.0g) respectively. Similarly, the maximum size of onion bulbs (62.5g), yield (281q/ha) was recorded in the hand weeded plots followed by the combined application of glyphosate, pendimethalin and oxyflurfen treated plot (61.9g and 268q/ha) as compared to weedy check (13.3g and 49q/ha). The highest cost benefit ratio was obtained in the treatment (where combinations of the 3 herbicides were applied i.e., (T7) due to low labour consumption as compared to weed free treatment (T1).