Abstract: The study was carried out in Government Seed farm Vavuniya from June 2012 to September 2012 by Random Completely Block Design (RCBD) with nine treatments and three replicates to study the role of Sebania rostrata in relation to soil properties and to evaluate the suitability of Sesbania and inorganic fertilizers on the yield of onion. The green manure (Sesbania rostrata and Crotalaria juncea (sunhemp)) were incorporated into the soil before 14 days of planting by manually. Onion bulb( Vethalam variety) was planted fourteen days after incorporation of organic material with spacing of 10 cm x 10 cm in a plot size of 1m x 1m. The treatments’ T2 – T9 plots were fertilized with urea, Sesbania rostrata and sunhemp and it’s combinations at the equivalent of 6.21g total N per m2 (Department Recommendation ). Plants were irrigated once in 4 days as farmer practiced.