Because of the valuable composition of buckwheat grains, a wide range of products containing those grains appeared on the market. The processes applied during the processing of those grains and the thermal treatment of buckwheat products can cause changes in significant components, which affect their nutritional properties. The objective of the study was to investigate changes in the nutrients and the digestibility of starch caused by buckwheat grain processing, i.e. by the preparation of flakes and roasted and unroasted groats, and in the case of groats, additionally, the effects of culinary processing carried out in two ways. It was showed that, compared to grains, buckwheat flakes contained less minerals and less proteins, whereas in the groats, this dependence was reversed. The roasting process impacted the composition of boiled groats; they contained less mineral compounds in the form of ash, less fat and less protein than the boiled unroasted groats. The method of boiling in water (with or without pouring out) did not cause significant differences in the protein content in the two types of groats and in the fat content in roasted groats. Roasting caused the trypsin inhibitor activity to considerably decrease and the content of non-protein nitrogen in buckwheat groats to increase. As regards the buckwheat grains, the processes applied in the production of buckwheat flakes and buckwheat groats resulted in a significant reduction in the amount of slow-digested starch and resistant starch because constituents were removed that limited the starch digestion and because of the hydrothermal treatment that caused the starch to gelatinize. After boiling, roasted groats contained about four times less resistant starch than unroasted groats. A high content of available thiol groups in buckwheat grains and buckwheat products, also after culinary treatment of groats, indicates their potential functional properties, e.g. antioxidant properties.