A field experiment was conducted at Jimma University College of Agriculture and Veterinary Medicine Research field during 2010/2011 in dry season to study the effects of Nitrogen, Phosphorus and Potassium fertilization on growth, yield, quality and storage life of irrigated Onion under Jimma conditions, Southwestern Ethiopia. The treatments consisted of factorial combinations of four levels of N as Urea, three levels of P as TSP and four levels of K as Potassium sulphate which were laid out in a Randomized Incomplete Block Design with three replications. Data on growth, yield, quality and storage life parameters were recorded and analyzed using GenStat 12.1 version computer soft ware packages. Results of the study revealed that; N, P and K had shown a highly significant difference on growth, yield, quality and storage life parameters.