Onion is one of the most important vegetable crops in Ethiopia as well as in Amhara region. However, the productivity of onion is markedly low due to soil fertility depletion and poor fertilizer management practices. Therefore, field experiment was conducted at Debre Zebit in 2019 under irrigation to investigate the response of onion to Nitrogen and phosphorus fertilizer rates. The treatments consisted of four levels of Nitrogen (0, 50, 100, and 150kg N ha-1) and four levels of phosphorus (0, 150, 200 and 250kg ha-1). The experiment was conducted in a 4× 4 factorial arrangement in a randomized complete block design with three replications. Results of the study showed that the main effect of N fertilizer significantly influenced plant height, number of leaves, leaf length, bulb maturity, bulb diameter, average fresh bulb weight, total yield, marketable yield and unmarketable yield. However, bulb length was not affected by the main effect of N fertilizer. The main effect of phosphorus fertilizer and its interaction with nitrogen fertilizer did not affect any of the growth and bulb yield parameters studied. However, the interaction effect of the two factors Nitrogen and Phosphorus influenced total bulb yield and fresh bulb weight. Increasing the rate of nitrogen fertilizer from 0 kg N ha-1 up to 100 kg N ha-1 increased both total and marketable fresh bulb yields significantly (P < 0.001) and linearly. However, increasing the rate of the fertilizer beyond 100 kg N ha-1 did not change the magnitude of both yields. The highest economic benefit (685257 Birr) was obtained at the combined application of 100 kg ha-1 Nitrogen and 200 kg ha-1 phosphorus and the highest marginal rate of return with the value of 435550.72 % recorded from the combined effects of 100 kg ha-1 Nitrogen and 200 kg ha-1 phosphorus. Based on the results of the present study, combined application of 100 kg ha-1 Nitrogen and 200 kg ha-1 phosphorus gave the highest net benefit of onion with acceptable marginal rate of return which can be recommended for economical production of onion in Debre Zebit areas and with similar agro-ecologies. As the results are limited to one season and location, further study over more seasons and locations both during the off and rainy periods is required to establish the forceful recommendations.