In order to study the effect of heat stress during GRAIN FILLING PERIOD on yield, yield components, GRAIN FILLING rate and duration, a field experiment was carried out during 2010-2011 growing season in Ahvaz conditions, as factorial based on randomized block with three replicates. The experimental factors consisted of 10 mid maturity bread wheat genotypes (Charnran, Atrak, Aflak, Dez, Falat, Darab2, Kavir, Pishtaz, S-78-11, S-83-3) and two sowing dates, including the optimum (12 Nov.) and late sowing date (21 Dec.). The results showed that in late sowing date, yield and yield components were reduced significantly in all genotypes. Atrak and Pishtaz had the minimum (8.2 %) and maximum (26.3%) reduction of GRAIN yield, respectively. In late sowing date, Kavir and Charnran had the highest (584.3 and 579.8 g.m-2, respectively) and Pishtaz had the lowest (446.3 g.m-2) GRAIN yield. Based on the heat susceptibility index among all genotypes, Atrak was heat- tolerant, Charnran, Kavir, Dez, Falat and Aflak were moderately tolerant and S-78- 11, S-83-3, Darab2 and Pishtaz were heat- susceptible. The shortening of EFFECTIVE GRAIN FILLING PERIOD in late sowing date resulted in reduction of thousand-GRAIN weight and increase in GRAIN FILLING rate could not compensate for .GRAIN FILLING PERIOD reduction. Overall, it seems that only the higher GRAIN growth rate under heat stress during GRAIN FILLING PERIOD cannot contribute to the higher GRAIN yield and other factors such as GRAIN FILLING PERIOD and number of GRAINs per spike seem to be more important.