The objective of the research study was to determine the effect of flaxseed oil cake, added to grass pea seeds as a tempeh-type fermentation co-substrate, on the antioxidant potential of the products produced. Five types of tempeh were made: one type made from grass pea seeds only and 4 types with the addition of 5, 15, 25, and 35 % of flaxseed oil cake. The products produced were lyophilized, defatted and, next, aqueous and aqueous-acetone extracts (1:1, v/v) were made, and the following was determined therein: content of phenols, antiradical activity (ABTS^+• and •OH or DPPH^•), and reducing power. As for the products containing flaxseed oil cake, the content of phenols in aqueous extracts was 20 – 80 % higher than in the tempeh from grass pea seeds only. Moreover, those extracts were characterized by a 50-100 % higher activity towards ABTS^+• radical. Additionally, the addition of only 15% – a relatively low amount – of flaxseed oil cake to the basic fermentation substrate caused a maximal increase in the •OH scavenging activity potential. The reducing power of aqueous extracts from products with 35% of flaxseed oil cake added was 40% higher and statistically significant compared to the tempeh from grass pea seeds. The content of phenols in aqueous-acetone extracts was 0.5 – 3.5 times higher in the products with flaxseed oil-cake added. Additionally, these extracts showed a 1.5 – 10 times higher activity towards DPPH^• and a 1.7- 3 times higher reducing power compared to the products produced after the fermentation of grass pea seeds only. The antioxidant activity was correlated with the content of phenols irrespective of the type of the used extract.