Studying the relationships between yield components gives an outstanding aid to understand the physiological basis of crop yield. A Field experiment was conducted to evaluate the yield and yield components of drought-tolerant and susceptible cultivars of wheat under drought stress, at the Ferdowsi University of Mashhad Research Field. Experiment was set up as a split plot design with 5 irrigation treatments (as main plots), 2 cultivars (as sub plots) with 3 replications. Irrigation treatments included irrigation at the rate of full crop water requirement (FI, first control), without irrigation during spring (NI, second control), irrigation at the rate of 75% crop water requirement (75%FI), irrigation at the rate of 50% crop water requirement (50%FI) and irrigation at the rate of 25% crop water requirement (25%FI), and cultivars also included Pishgam (drought-tolerant) and Gascogne (drought-susceptible). Results showed that the differences among irrigation treatments and varieties were significant for yield components, economical and biological yields. The number of ear.m-2 and number of grain.ear-1 were decreased by reducing water supply, while 1000-grain weight was increased. Raise of 1000-grain weight could not compensate for the reduction of ear and grain number, therefore, economical and biological yield was decreased by reducing water supply. Harvest index was also decreased by reducing water supply. Overall, the tolerant cultivar showed a better performance in terms of economical yield, biological yield and water use efficiency comparing with the susceptible one, but it had lower harvest index than susceptible cultivar. Results showed that retention of greater ear number per unit area and greater grain production per ear were the most important prominence factors of tolerant cultivar comparing with the susceptible one. The efficacy of FAO Penman-Monteith method in evaluation of wheat water requirement was also assessed reliable.