Onion detopping operation is tedious, time consuming and labour intensive which increases cost of operation. About 12.5 man-hours are required in manual detopping operation of 1 MT onion bulbs. The onion detopping is calls for its mechanization to reduce the losses in field and for timeliness in operation, fetching good market price and better storability. Therefore, a power operated onion detopper was developed for reducing the drudgery and labour involved in onion detopping operation. It consists of main frame, conveying unit, cutting unit, safety guard and power transmission unit. Electric motors were used as prime movers for conveying and cutting mechanism. The performance of developed onion detopper was evaluated. The average de-topping efficiency was observed as 86.10 per cent with 305.68 kgh-1 output capacity. The power requirement was found to be 0.5 kW h-1. Average time required for detopping is 0.327 h q-1. Average onion neck length of 21.09 mm was found which is sufficient for reducing storage losses of onions. The cost of operation was worked out to be Rs. 21.26 per quintal.