This study analysed the influence of technological processes used in the various buckwheat groat mills on the phenolic content and antioxidant activity of common buckwheat grains and groats. Dehulling, to a greater extent, influenced the quantitative changes in rutin and caused its lowering. Due to this process, the antioxidant activity decreased to a lower degree, and the content of most phenolic acids increased. The roasting process had a negative impact on the content of phenolic compounds, causing the greatest loss in the level of coumaric acid and rutin. It was found that in shaping the health benefits of buckwheat groats, the grain processing temperature is more important than the time of the treatment. In order to obtain the best health value of buckwheat groats, the roasting process should be carried out at a temperature close to 100 °C for a time that will ensure good sensory characteristics, even if it is for several hours.