The effect of high temperatures (above 25°C) on starch concentration and the morphology of starch granules in the grains of wheat (Triticum aestivum L.) were studied. Wheat plants of cultivars Yangmai 9 (weak-gluten) and Yangmai 12 (medium-gluten) were treated with high temperatures for 3 days at different times after anthesis. The results showed that the starch concentration of grains given a heat-shock treatment above 30°C were lower than those developing at normal temperature in both cultivars. High temperature lowered starch concentration due to the decrease of amylopectin. Under the same temperature, the effect of heat shock from 6 to 8 days after anthesis (DAA) was the greatest, whereas from 36 to 38 DAA the effect was the least. The effects of high temperatures after anthesis on starch-pasting properties were similar to those on starch concentration, especially after 35-40°C treatments. The size, shape and structure of starch granules in wheat grains (determined by electron microscopy) after heat shock were visibly different from the control. When given heat shock during development, the starch granules in mature wheat grains were ellipsoid in shape and bound loosely with a protein sheath in Yangmai 9, while they were damaged and compressed with fissures in Yangmai 12, indicating the differences in resistance to high temperature between cultivars. Ratios of large (type-A) and small (type-B) starch granules significantly decreased under heat shock, which limited the potential sink size for dry matter deposition in the grain.