Onion (Allium cepa L.) is one of the most important bulb crops in India, which plays a major role in supplementing the income of small and marginal farmers of Samastipur, Vaishali, East Champaran, Buxar, Begusarai, Nalanda and Bhojpur District in Bihar state. One of the major constraints of kharif onion farming is poor nursery and low productivity due to non adoption of recommended package of practices and improved verities. To replace this anomaly, National Horticultural Research and Development Foundation, Patna (Bihar) had conducted TDTD (Technology Dissemination through Demonstration) under National Horticulture Mission, Govt. of India, at adopted farmers’ fields. Cultivation practices comprised under TDTD viz., use of improved variety/season specific, Nursery Raising, Soil/Seed treatment, Transplanting, fertilizer application and control of Purple Blotch disease, showed that percentage increase in the yield of Onion ranged from 16.09 % to 26.17 % over local check during the course of study from 2013-14 to 2018-19. The technology gap of 40.8 q/ha during 2018-19 from 77.5 q/ha at the initial stage of study (2014-15) shows the gap in demonstration yield over potential yield, but the above gap reduced subsequently in the following years.