In order to evaluate the relationshipbetween yield and some relevant traits and estimation of the most effective traits on grain yield,a split-plot experiment based on completely randomized block design with four replications was carried out in 2006–2007 in the research field of Islamic Azad University of Karaj. The irrigation schemes scheduled as main plots included the following: (T₁) 40% moisture depletion throughout the growing season (control); (T₂) 60% moisture depletion throughout the growing season; (T₃) 80% moisture depletion throughout the growing season; (T₄) no irrigation during the stem elongation stage and continuing with adequate irrigation to the end of the growing season; (T₅) no irrigation from the stem elongation stage to the end of the growing season; (T₆) no irrigation at flowering and continuing with adequate irrigation to the end of the growing season; (T₇) no irrigation from flowering to the end of the growing season; and (T₈) no irrigation from the milk stage to the end of the growing season; and 2 wheat cultivars [Marvdasht (V₁), Chamran (V₂)] as sub-plots. According to the results of simple correlation, the grain yield exhibited the most positive correlation with biomass (0.877), number of infertile spikelets (0.876) and harvest index (0.855). The results of step-wise regression showed that, in the absence of drought stress, biomass and harvest index had respectively the most important effects on the grain yield but both exhibited different results under drought stress. Path analysis results showed that the most important effect on the grain yield in the total tested treatments was related to the biomass, justifying a total of 87% of grain yield variations, 72% of which is the direct effect of this trait on grain yield.