The duration of grain filling period and the rate of this period determines final grain weight in barley. In this study, in order to investigate the effect of final drought stress on rate and duration of grain filling and contribution of stem reserves in main spike yield, a trial was conducted as split-plot design with three replicates (blocks) at Shahid Chamran University in the duration of 2010-2011. The main plot consisted of two irrigation treatments (well-watered and droughted treatment after anthesis) and the split-plot consisted of nine barley cultivars. The results showed that drought had no significant effect on the effective duration of grain filling but reduced the rate of this period by 20%. There was a significant difference between cultivars considering duration of grain filling but they did not show any difference in the rate of grain filling period. Final grain yield ranged between 251 to 536 (g/m2). 25 percent decrease of grain yield by entering drought conditions is almost due and 19 percent decrease in grain weight, because other yield components did not show any significant changes. The yield of main spike showed 24 percent decrease by entering drought conditions. The contribution of stem reserves in the yield of main spike increased by 33% under drought conditions and did not show any significant correlations with maximum weight and length of stem in none of moisture treatments.