Onion (Allium cape L.) is an important vegetable crop commonly produced both for home consumption and income generation by small scale farmers and commercial growers in many parts of Ethiopia. However, the crop yield in Raya Kobo district is much lower than the crop potential due to inappropriate agronomic practices and lack of access to adaptable onion varieties. A field experiment was conducted at Raya Kobo district during 2021 irrigation season to evaluate the effects of time of N fertilizer application on the growth, yield and yield components of onion varieties. The treatments were four times of N fertilizer application; TF1 (15 and 45), TF2 (10, 25, 40), TF3 (10, 30 and 50) and TF4 (10, 35, and 60) after date of transplantation and four onion varieties (Adama Red, Bombay Red, and Nafis and Nasick Red). The experiment was conducted in a 4×4 factorial arrangement in a randomized complete block design with three replications. Data were recorded and subjected to ANOVA using SAS 9.0 Software. Results showed that the interaction of time N fertilizer application and variety of onion significantly affected plant height, leaf length, maturity date, aboveground fresh weights, total biomass, bulb weight and diameter, marketable yield and total bulb yield of onion. On the other hand, the main effects of time N fertilizer application significantly influenced leaf number, and onion varieties highly significantly influenced unmarketable yield. The tallest plant height (63.47cm), leaf length (56.03cm), the largest aboveground biomass (5.24 tones/ha), the largest total fresh biomass 38.94 tones/ha and the highest average bulb weight (89.47 g), marketable bulb yield (35.19 t/ha), and total bulb yield (36.34 t/ha) were recorded from variety Nasick Red supplied with N fertilizer at 10, 35 and 60 DAT with the rate of 46kg/ha N fertilizer split in three times equally. Based on the results, Nasick Red variety of onion supplied with nitrogen fertilizer at the rate of 15.3 kg/ha at 10,35 and 60 days after transplanting, for each can be recommended for the study area as this combination recorded the highest net benefit (439254 Eth-Birr) with acceptable marginal rate of returns (11894.23%). To come up with precise and reliable recommendations, study should be repeated over seasons and locations with increase intervals.