Onion is one of the most important bulb crops of the Nalanda district. The trial is conducted to reduce the bolting percentage and improve the yield by transplanting onion seedlings at the proper age. Farmers are usually transplanting 75 to 80 days old seedlings in the Nalanda district. In this trail, farmers transplanted 60 days and 55 days old seedlings for consecutive two the financial year 2018-2019 and 2019-2020. The best result was obtained in the case of transplanting 55-day-old seedlings. The maximum bulb weight was 58.86 gm, the minimum bolting percentage was 1.98%, maximum yield 324.63 q/ha with the highest B:C ratio of 02.08 was recorded in the Year 2018-2019 in the case of 55 days of seedlings. A similar result was obtained in the year 2019-2020 as the highest bulb weight was 61.01 gm, the least bolting percentage was 1.98%, maximum yield was 325.25q/ha with the highest B:C ratio of 2.10 as compared to transplanting of 60 days old seedlings and farmers practice. In the case of farmers' practice, the average bulb weight was 57.45gm with a bolting percentage of 6.74%, the yield is 297.28 q/ha with B: C ratio of 1.77 in Years 2018-19, and 57.66 gm of bulb weight, 6.53% of bolting,296.56 q/ha of yield with B: C ratio of 1.76.