The onion detopping and grading 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-cum-grader machine was developed for reducing the drudgery and labour involved in onion detopping and grading operation. Power operated onion detopper-cum-grader machine was developed which consist of feeding mechanism, detopping mechanism and grading mechanism. Feeding mechanism consists of flat belt conveyor with 270angle of inclination and 0.59 ms-1 belt speed. Detopping mechanism consists of inward motion of spiral rollers and cutting unit. The peripheral speed of spiral rollers and cutters was kept 266 rpm and 1821 rpm respectively at load condition. Grading mechanism consists of two divergent rollers made of PVC pipe and having outward motion of 225 rpm with 70 slopes. Automatic detopping and grading is done simultaneously by this machine. The performance of developed machine 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. Onions after detopping are graded in five grades i.e. <35 mm, 35-50 mm, 50-60 mm, 60-85 mm and >85 mm. The average onion neck length before and after detopping was 314.8 mm and 23.43 mm respectively. Percentage of grading onions in grades <35 mm, 35-50 mm, 50-60 mm, 60-85 mm, >85 mm were 1.88 percent, 44.73 percent, 33.08 percent, 20.29 percent and 0 percent respectively.