The objective of this study was to evaluate the cultivar Dukat of covered oat, grown as a pure culture or with spring vetch as the undersown plant (30 vs 50 kg/ha), in a mountain region, in 1996 and 1997. Grain yield, protein yield, hulls contribution, and the mass of 1000 grains were determined. Also, gross chemical composition, fiber fractions, and amino acid composition of oat grain were analysed. Biological value (BV) and true digestibility (TD) were evaluated in balance trials on rats. Grain yield was affected significantly (P < 0,05) by both the undersown plant and growing seasons. The highest yields were obtained from oat grown as a pure culture. Undersowing spring vetch decreased grain and protein yields of oat. Oat ’97 had better chemical composition and improved nutritive value. Growing oat with spring vetch affected both composition and oat grain quality. It resulted in significantly (P < 0,01) lower hulls content and higher mass of 1000 grains, in two years of the study.