Complex plant extracts rich in polyphenols are being increasingly used as bioactive ingredients in the formulation of functional food and nutritional supplements, due to the wide spectrum of their biological effects, primarily, antioxidant. . In this paper, antioxidant activity and gastrointestinal stability of polyphenols from different formulations of grape seed extract and Polygonum Cuspidatum extract were investigated. The goal was to establish which extracts were suitable for the further use in designing polyphenol-enriched functional foods. To determine the potential of analysed extracts as nutraceuticals, both, their antioxidant activity and gastrointestinal stability need to be taken into account. Antioxidant activity of analysed samples was investigated by Folin-Ciocalteu (total reduction potential) and TEAC method (antiradical efficiency). Gastrointestinal stability/bioaccessibility of polyphenols in analysed extracts was determined by in vitro static simulation of digestion according to the optimized INFOGEST protocol Different formulations of grape seed extracts were comparable to each other in terms of antioxidant potential, while the analysed Polygonum cuspidatum extracts differed significantly. The antioxidant potential of the Polygonum cuspidatum extract was significantly lower in comparison to antiradical potential of the grape seed extract. It was determined that polyphenolic plant extracts were stable under simulated conditions of the digestive tract; the small decrease of the polyphenolic bioaccessibility was observed only in one analysed formulation of grape seed extract.