One-factorial experiments were conducted in 1996 and 1997 in order to compare grain, fat yield and fatty acid composition of oat cv. Dukat, grown with undersown spring vetch cv. Kwarta (0, 30, 50 kg/ha). The experiment, using cv. Dukat, was conducted in the mountains, in Czyrna near Krynica, on a mountain oat-potatoes complex (no 12). Grain fat content was determined according to the standard AOAC method, and fatty acid composition using a Varian 3400 gas chromatograph. All data were evaluated by analysis of variance and LSD (0,05) values were calculated. The yield of cv. Dukat, grown as a pure culture, was significantly higher than that obtained from cv. Dukat grown with spring vetch. The sowing techniques had no apparent effect on fat content in oat grain, but affected the content of saturated, monounsaturated, and polyunsaturated fatty acids in oat grain. Culivar Dukat grown with spring vetch showed lower content of saturated and higher content of polyunsaturated fatty acids.