Under subtropical conditions such as the south west of Iran, heat stress after anthesis is the major grain yield limiting factor in winter sown wheat genotypes. In order to study the effect of post-anthesis heat stress on grain yield, source restriction and the pattern of assimilate redistribution to grains under different nitrogen levels, two separate field experiments were conducted at delayed and optimum sowing dates in Ahvaz, Iran during 2007 and 2008 seasons. The experimental site had a moderate winter and dry, hot summer. Plants with delayed sowing date experienced heat stress post-anthesis. Each split-polt experiment had a randomized complete block design with three replicates. The N application rates were (50, 100, and 150 kgNha-1) assigned in the main-plots. Sub-plots consisted of six bread and durum wheat genotypes. Results indicated that, that the grain yield (GY) was reduced in the 50 and 100 kgNha-1 compared treatments with 150 kgNha-1 treatment. Compared with optimum conditions, heat stress after anthesis reduced grain yield and grain weight 24% and 31%, respectively. GY reduction under post-anthesis heat stress conditions was due to significant grain weight reduction. In all genotypes, the source restriction (SR) reduction under post-anthesis heat stress conditions was 45% compared with the non-stressed treatments. The highest and the lowest SR increment under post-anthesis heat stress conditions was belonged to D-84-5 and D-83-8 lines, respectively. The SR increment in long, middle and short season genotypes under stressed conditions was 56%, 43.5% and 30.5%, respectively. Although, the assimilate redistribution increased under heat stress condition and this increment was higher in long season genotypes, but significant reduction of the current photosynthesis and the rate of redistributed dry matter to grains and also increment of the SR, leaded to 1000-grain weight and GY reduction.