Onion is one of the most important vegetable crops in Koga Irrigation Scheme which is mainly produced by small holder farmers. However, the yield of the crop is constrained by several factors. Limited availability of improved varieties and inappropriate application of nitrogen fertilizer are among the major constraints of onion production. Therefore, a field experiment was conducted at Koga Irrigation Scheme during the 2022 Irrigation season to evaluate the effects of nitrogen fertilizer on the growth, yield and quality of hybrid onion varieties and to determine the optimum nitrogen fertilizer for economical production of onion varieties. The treatments consisted of four onion varieties (Red coach, Russet, Jambar and Bombay red) and four levels of nitrogen (0, 41, 82 and 123kg ha -1 ). The experiment was laid out in Randomized Complete Block Design in a factorial arrangement with three replications. Growth, yield and quality parameters were collected and the analysis of variance for the collected data was performed by using SAS (version 9.4) software. The results of the study indicated that the main effect of varieties and nitrogen fertilizer very highly significantly affected stand count, days to bulb maturity, plant height, leaf length, leaf number, bulb neck diameter, bulb length, harvest index, total soluble solid and bulb dry matter contents. The interaction effects of varieties and nitrogen fertilizer highly significantly influenced the bulb weight, bulb diameter, marketable bulb yield and total bulb yield of onion. The highest marketable bulb yield (57.84 t ha ), bulb weight (165.87g) and bulb diameter (7.89cm) was obtained from the interaction of Russet variety with 82 kg N ha -1 . However, the lowest marketable yield (21.74 t ha ) was obtained from Bombay red variety with control treatments. Based on the partial budget analysis, the highest net benefits (969884.8 Birr ha -1 ) and the highest marginal rate of return (2007.50%) were recorded from Russet variety with 82 kg ha vi -1 nitrogen fertilizer. Therefore, Russet variety with 82 kg N ha -1 is recommended for the production of onion for Koga Irrigation Scheme. As the results are limited to one season and location, further studies over multiple locations and seasons are necessary to establish sustainable recommendations.