Eleven wheat cultivars were evaluated at 10-day intervals, beginning from anthesis, under irrigated and drought stress conditions during 2006-2007. The effects of irrigation, genotype and date of harvest were significant for most of the studied characters. Water deficit decreased pre-and post-anthesis assimilation rate, grain weight per spike, grain number per spike and 1000 grain weight about 5.7, 24.5, 21.2, 15.7 and 6.4 %, respectively. Mobilization, mobilization efficiency and contribution of pre-anthesis assimilates to kernels were considerably increased under drought stress condition. grain weight, grain growth rate and contribution of current assimilates to grain filling decreased under drought stress about 7.18, 22.1 and 29.6 %, respectively. However, the effective grain filling period was considerably increased in the stressed plants. grain filling rate was correlated with the accumulation of dry matter at maturity, grain weight per spike and grain number per spike in the irrigated and drought stressed environments (r=0.87** and 0.53, r=0.87** and r=0.62*, r=0.75* and r=0.63*, respectively). A negative correlation was found between effective grain filling period and grain yield/spike under irrigation (r =-0.65*) and drought stress (r =-0.76**) conditions. Furthermore, positive correlations between grain filling rate and grain yield were obtained in the irrigated and drought stressed environments (r = 0.87** and r = 0.62*, respectively). It seems that accumulation of pre-anthesis assimilates (mainly under drought stress), short effective grain filling period and high grain filling rate are major factors for producing higher grain yield in wheat under both irrigated and drought stress conditions.