The impact of suppressing the caffeic acid O-methyltransferase (COMT) gene on lignin and fiber content as well as seed oil composition of Brassica napus was studied. Transgenic lines showed significant reduction in COMT enzyme activity ranging between 21 and 31 % residual activity. Lignin in the transgenic Cruciferin:COMT line 34-3 seeds was reduced by 35 % of the wild type, which decreased acid detergent fiber and neutral detergent fiber by 17.92 and 13.04 % of the control, respectively. The main fatty acids were monounsaturated (67.2–69.5 %) followed by polyunsaturated (20.9–23.3 %). The content of tocols ranged between 338 and 440 mg/100 g oil. The highest antiradical action toward 1,1-diphenyl-2-picrylhydrazyl and galvinoxyl radicals was in the oil of the transgenic line 34-3, which contained the highest amount of total phenolic compounds and total tocols. The results show the possibility of reducing lignin in B. napus with no effect on oil content and quality.