Field investigations were conducted for two years at Agricultural Research Station, Bhavanisagar of Tamil Nadu Agricultural University, during kharif season of 2009 and 2010 to evaluate the new formulation of oxyfluorfen (23.5% EC) on weed control in onion. Ten weed control treatments were tested in randomized block design replicated thrice. All the treatments significantly reduced the population and dry weight of weeds which resulted in significantly higher growth and yield of onion over unweeded control. Application of new formulation of oxyfluorfen (23.5% EC) at 200 g ha-1 as pre-emergence herbicide can keep the weed density and dry weight below the economic threshold level and also increase the bulb weight (42.56 and 43.87 g) and yield (15940 and 15610 kg ha-1) in onion. During both the years, the application of new formulation oxyfluorfen at 400 g ha-1 registered lower values of nitrogen, phosphorus and potassium removal by weeds. Among the weed management practices, pre-emergence application of oxyfluorfen at 200 g ha-1 resulted in higher net return of ` 3,87,007 ha-1 and ` 3,75,657 ha-1 and benefit cost ratio of 5.24 and 5.05 during both the year of study, respectively and also increased the bulb yield of onion with the concomitant increase in the yield attributes and also resulted in higher net returns.