Common buckwheat (Fagopyrum esculentum Moench) is a valuable crop plant with cereal-like seed chemical composition; however, it is susceptible to thermal stress. The aim of the study was to determine whether heat-shock proteins HSP-90 and HSP-70 can protect common buckwheat against thermal stress during development of microspores and embryo sacs. The study was performed on two accessions of common buckwheat, Panda and PA15, which differed in their tolerance to thermal stress. Accumulation of these proteins was determined in buds, open and wilted flowers, and donor leaves of plants grown at 20°C (control) and 30°C (thermal stress). Photochemical efficiency of donor leaves, closest to the inflorescences, based on chlorophyll a fluorescence (ChlF) was also analysed. All plants demonstrated higher values of ChlF at 30°C than at 20°C, which suggests that this 30°C temperature is more conducive to their vegetative growth. Pollen grains of both accessions demonstrated normal development at 30°C, whereas embryo sacs showed many developmental disturbances. Panda was more sensitive to thermal stress than PA15, as manifested in a higher percentage of degenerated embryo sacs at the flower bud phase. Moreover, a decrease in both HSPs in the studied organs of Panda was found relative to the control. At 30°C, both accessions accumulated more HSP-70 than HSP-90. These results suggest that, under heat stress, HSP-70 plays a protective role for flowers of common buckwheat. The analyses indicated that the donor leaf closest to the flower cluster may be a reliable indicator of temperature sensitivity in buckwheat flowers.