A field experiment was conducted at the IPA Field Crops Research Station, Abu – Graib during 2002 season and at Experimental Field, Department of Field Crop Science, College of Agricultur , Abu – Graib during 2004 season. The objective of this research was to study the effect of six levels of foliar potassium application 500 ppm at start of flowering and 500 ppm at start of boll development, 1000 ppm at start of flowering and 1000 ppm at start of boll development, 1500 ppm at start of flowering and 1500 ppm at start of boll development, 2000 ppm at start of flowering and 2000 ppm at start of boll development, 2500 ppm at start of flowering and 2500 ppm at start of boll development, 3000 ppm at start of flowering and 3000 ppm at start of boll development in additional to control treatment (without spray) on seed cotton yield, lint yield and lint quality of cotton var. Cocker 310. A randomized complete block design was used with three replications. The results indicated that highest response of Cocker 310 c.v . to increasing potassium foliar application was by 3000 ppm at start of flowering and 3000 ppm at start of boll development, it gave asignificant increasing in the number of sympodia, number of open bolls, boll weight, seed cotton yield, dry matter ,lint yield and fiber strength as compared to control in both seasons. While seed , number/boll, fiber length and fiber fineness did not significantly affected by all the level of potassium applied on the two season.