An experiment was conducted to find out the combined effect of cowdung and potassium on the growth and yield of onion cv. BARI piaz-I at Horticulture Farm, Bangladesh Agricultural University, Mymensingh during the period from December 2010 to March 2011. The experiment was laid out in a randomized complete block design with three replications. The two factors experiment had four levels of cowdung, viz., 0, 5, 10 and 20 tons ha-1 and four levels of potassium, viz. 0, 50, 150 and 250 kg K ha-1 . Doses of cowdung and potassium showed significant variation in respect of all the parameters studied. The combination of 10 tons cowdung and 250 kg K ha-1 gave the tallest plant (46.60 cm), the highest number of leaves plant-1 (6.40), the highest length of bulb (3.27 cm), the highest diameter of bulb (4.83 cm), individual weight of bulb (51.23 g), dry matter content (12.66%) and yield of bulb ha -1 (12.83 tons); whereas the control treatment gave the shortest plant (38.15 cm), lowest number of leaves plant-1 (5.68), diameter of bulb (3.41), individual weight of bulb (35.65g) and gave lowest bulb yield ha-1 (9.16 tons).