In onion, the extent of heterosis for yield and associated characters was studied during kharif 2004-05, in a set of 30 hybrids produced from a Line x tester mating design involving five lines and six testers of diverse nature maintained in pure form in the Division of Vegetable crops, IIHR, Bangalore, India. Appreciable amount of heterobeltosis and standard heterosis was noticed for all the traits studied. The crosses PBR 139 x AN 184 and PBR 140 x AN 187 recorded high heterosis of 45.31 and 32.83 per cent over better parent and 27.40 and 31.01 per cent over standard check ( Arka Kalyana ) respectively for marketable bulb yield and were identified as the best hybrid combinations for exploring hybrid vigour. These two crosses also recorded positive and significant heterobeltosis and standard heterosis for other characters studied.