Wheat in the Mediterranean zone often encounters high temperatures during the terminal growth stage. This study investigated whether assimilate supply by source-function plays a dominant role in determining grain production in spring wheat exposed to high terminal-temperatures in the Mediterranean zone of south-east Turkey. The spring wheat cultivar Adana99 was grown under irrigated conditions sowing according to the current schedule (CS plants) and late-sowing (IS plants) for two years. Grain yield of CS plants, which were subjected to relatively lower temperatures during the terminal growth stage, was higher than that of IS plants, which were subjected to relatively higher temperatures. A high temperature in the post-anthesis period accelerated leaf senescence and reduced radiation-use efficiency. Halving the plant density at anthesis, which increases assimilate supply to grains, significantly increased the grain dry weight (GDW) and whole plant dry weight (WPDW) in the post-anthesis period. Grain filling percentage (F%, observed/final GDW) in thinned plants, which was regarded as potential F%, showed a single logistic equation based on cumulative temperature after anthesis, regardless of post-anthesis temperatures. In the LS plants, the daily increase in WPDM (Delta WPDW) decreased, but the peak of an increase rate of potential GDW estimated from F% was shifted to earlier-filling stage, resulting in reduced GDW at harvest. The final GDW in both years closely correlated with the estimated Delta WPDW in the grain-filling period. These results suggest that source-function for assimilate supply in the post-anthesis period is one of the important yield-determining processes in spring wheat subjected to high temperatures.