abstract: Fatty acid composition of muscle triaciloglicerols and micro morphological structure of m. longissimus thoracis et lumborum and m. semimembranosus were determined. Linolenic acid (C18:3) level in m. longissimus triaciloglicerols significantly (0.5%, 1.2% and 2.1%) increased for I, II and III group respectively. Omega ratio expressed as ratio C18:2/C18:3 in fat at m. longissimus was 9.0, 4.7 and 2.3 respectively for the three groups. Linolenic acid level in fat at m. semimembranosus rose significantly (0.4%, 1.8% and 2.6%) and omega ratio was 15.75, 3.78 and 2.58 respectively for I, II and III group. No statistically significant differences were found in mean diameter of muscle fibers, in ratio between dark and light muscle fibers and content of different tissue components in studied muscles caused by including of flax seed in the diets.