A field experiment was carried out on a clay loam soil at Fakous district, Sharkia Governorate, Egypt during the summer growing season of 2001 to study the effect of different potassium application methods and levels on cotton growth, yield and yield components, fiber quality and some chemical constituents of cotton plant (Giza 85). The experimental treatments were seven and replicated four times in a randomized complete blocks design. The treatments included unfertilized check, three rates of potassium-soil application (24,48 and 72 kg K2O / fad) and three rates of foliar spraying of K using a commercial liquid fertilizer named POTASSIN (750, 1000 and 1250 cm3/fad). The major findings can be summarized as follows:               Applying K with both methods increased number of fruiting branches/plant, chlorophyll contents (a, b and total), seed protein, oil contents, number of opened bolls / plant, boll weight, seed cotton yield, earliness and lint % while, the reverse was true for position of first fruiting node, carotenoid content and carbohydrates content in leaves (reducing, non reducing and total soluble sugars). K-application had insignificant effect on plant height at harvest, seed index and fiber properties. Cotton plants exhibited higher response to foliar feeding than soil potassium fertilization especially, when plants sprayed by the highest level of POTASSIN. Therefore, it could be concluded that potassium fertilization can be used for promoting cotton plant growth and yield.