Onion is one of the most important vegetable crops produced as a condiment for home consumption and income sources in many parts of Ethiopia. However, productivity of onion is low due to various limiting factors such as low soil fertility, improper plant spacing, disease and pests, lack of site specific variety and management practice. Rates of fertilizer application and intra row plant spacing are among others the most important once. Therefore, this study was initiated to evaluate the effects of NPS fertilizer and intra row plant spacingon growth, and bulb yield of onion at Mekdela district under irrigation condition in 2019/2020 cropping season. The treatments were consisted of four levels of NPS (0, 50, 100, 150 kg ha-1) and three levels of intra row plant spacing (5cm, 10cm, 15cm), which were factorial combined and laid out in Randomized Complete Block Design with three replications. Growth and yield components were collected following the respective standard procedures and analyzed using SAS software version 9.3. Analysis of variance revealed that application of NPS fertilizer combined with intra row plant spacing highly significantly (p≤0.01) influenced most of the tested parameters of onion including days to 75% physiological maturity, plant height, leaf number, bulb neck thickness, diameter, length, weight of onion bulbs, marketable, unmarketable and total bulb yields. Accordingly, the treatment combination of 150 kg ha-1 NPS fertilizer and 10cm intra row recorded the highest plant height (67.74cm), bulb diameter (7.01 cm), bulb length (5.85cm), average bulb weight (139.05 g), marketable (41.58 t ha-1) and total bulb yields (42.04 t ha-1). In the correlation analysis, almost all the tested parameters of onion were highly significantly and positively correlated with marketable bulb yield of onion. Plants supplied with 150 kg ha-1 NPS and planted at 10 cm intra row spacing recorded the highest net benefit(Birr 930150.00) with acceptable MRR (8225.00%), which can be recommended for economical production of onion in Mekdela district. As the present study was conducted at a single season and location further research work at multiple seasons and locations is also recommended. Keywords; Blended fertilizer, bulb weight, bulb yield, onion, spacing