The present study was stimulated by the problem of onion seed production where seed production is possibly only winter period after which rapid increase in temperature as well as early shower adversely affect the quality of seed. The experiment was laid out in a randomized complete block design with four replications to determine the effect of bulb size on the yield and quality of onion seeds. Three treatments viz. B1= 10 g, B2= 15 g and B3= 20 g were consisted to achieve the objective. The research work was done at On-Farm Research Division, BARI, Bogra during September, 2011 to June, 2012. The results revealed that the growth parameters, seed yield parameters, health and quality of harvested seeds were significantly influenced by the different treatments. Results showed that among different treatments, optimum bulb size is 15 g for production true seeds of onion at Bogra region in Bangladesh.