The effects of drought stress and potassium foliar application on grain yield and some physiological traits of three wheat (Triticum aestivum L.) cultivars, was studied through field experiments conducted during growing season of 2011-2012 at two locations, 1. Seed and Plant Certification and Registration Research Institute (SPCRI), Karaj- Iran 2. Agricultural and Natural Resources Research Center Yazd- Iran. The experiments were carried out using a split plot factorial based on a randomized complete blocks design with three replications. The experimental treatments included three irrigation regimes; normal, mild stress (water withheld at the grain filling phase) and severe stress (water withheld at the ear emergence phase); potassium foliar treatments included, 0, 1.5% and 3.0% K2O applications and three wheat cultivars (Marvdasht, Pishtaz and WS-82-9). Leaf stomatal conductance, transpiration rate, chlorophyll, biological yield and grain yield were measured. The results indicated that water stress through withholding at the ear emergence and grain filling phases reduced the above mentioned traits at the both locations. Also these traits showed significant increase by increasing potassium foliar application.