Onion is one of the most important high market value bulb crops cultivated commercially in most parts of the world. It is the most cultivated and high market value of vegetable crops In Afghanistan specially Ghazni. However, bulb yield of onion is limited due to improper use of plant spacing. Therefore, this study was held to determine the optimum planting spacing of Onion to maximize its productivity in 2016 and 2017 under irrigation conditions. The trial contains treatment of singe planting pattern of 20 x 10, 40 x 10, 30 x 5, 30 x 7.5, 30 x 10 and 30 x 12.5 cm inter and intra spacing respectively and double planting pattern combinations of four intra plant spacing (5, 7.5,10 and 12.5 cm) and two levels of inter spacing (40 and 50 cm) by means of total fourteen treatments were carried out in Factorial Randomized Block Design (FRBD) with three replications. The current findings showed that plant height, bulb size, bulb weight, total bulb yield and marketable bulb yield were significantly influenced by planting spacing. Accordingly, the tallest plant height (37.70 cm) was obtained at a spacing of single planting of 40 cm inter row and 10 cm intra row while the maximum total bulb yield of 27.01 ton ha-1) was recorded at a spacing of double planting pattern of 40 cm inter and 5 cm intra row in both years. Therefore, 40 cm inter row and 5 cm intra rowspacing in double row planting manner is recommended for the growers to improve onion productivity in the study area.