Abstract: Field experiments were conducted in 2003/2004 and 2004/2005 dry seasons to study the effect of Nitrogen (N) and Phosphorus (P) on the growth and yield of irrigated onion in the Sudan Sananna of Nigeria. The treatments consisted of factorial combinations of four levels of nitrogen (0, 50, 100 and 150 kg N ha-1) and three levels of phosphorus (0, 17.5 and 35 kg P ha-1) laid out in split-plot design with three replications. Nitrogen was allocated to the main plots while phosphorus was assigned to the sub-plots. Results revealed that N and P as well as their interaction, significantly affected plant height, number of leaves per plant, percentage bolters, crop growth rate and individual bulb weight. However, interaction was not significant on bulb yield. Nitrogen at the rate of 150 kg N ha-1 gave the best results, though, statistically at par with 100 kg N ha-1. 17.5 kg P ha-1 gave statistically similar results as 35 kg P ha-1. The optimum combination from the results of this investigation was 100 kg N ha-1 and 17.5 kg P ha-1.