The research study the effect of potassium and row spacing on yield of onion was conducted at Dargai Malakand during summer 2012. The experiment was conducted in Randomized Complete Block Design with split plot arrangement having three replications. Potassium levels (0, 40, 80 and 120 kg ha-1) were applied to main plot while row spacing (15, 20 and 25 cm) were kept in sub plot. Recommended doses of nitrogen were applied at two split doses and phosphorus and potassium were applied at time of planting. The studied perimeters were plant height, number of leaves plant-1, bulb diameter, average bulb weight, numbers of bulb kg-1 and yield (t ha-1). The maximum plant height (51.6 cm), number of leaves plant-1 (9.89), bulb diameter (5.93), average bulb weight (64.89 g), leaf width (1.33 cm) and yield (22.91 t ha-1) were observed with the application of 120 kg K2O ha-1. The maximum number of bulb kg-1 (15.78) was observed in control. On the other hand the maximum plant height (47.1 cm), number of leaves plant-1 (8.67), bulb diameter (5.64 cm) and average blub weight (54.92 g) were recorded at 25 cm row spacing. And maximum leaf width (1.26 cm) and yield (22.45 t ha-1) was observed at 20 cm row spacing. The maximum bulb kg-1 (16.92) was observed at 15 cm row spacing. The highest yield were observed with the application of 120 kg ha- 1potash application and at 20 cm row spacing. Based on the above result it is recommended that 20 cm row spacing with 120 kg K2O ha-1 should be used for best growth and maximum yield of onion under the agro climatic condition of Dargai at Malakand