A field experiment was conducted to evaluate the effect of cobalt on cucumber growth, fruits yield and mineral composition. Seedlings of five weeks-old with almost the same stem thickness were transplanted to the field under drip irrigation system. Cobalt was added once in the form of cobalt sulphate in five concentrations namely, 0.0, 7.5, 15.0, 22.5 and 30.0 ppm. The obtained results indicated that the addition of 15.0 ppm cobalt had significant primitive effect on cucumber growth, fruits yield and mineral content. Higher concentrations exerted hazardous effect on vegetative growth, yield and Mn, Zn and Cu in fruits. Cobalt contents in fruits was increased with every increase in cobalt application. On the other hand, opposite trend was observed for Fe. Cobalt level of 15.0 ppm increased fruits N, P and K content but further increases in cobalt      addition up to 30 ppm decreased N, P and K in cucumber fruits.