Abstract: The effects of flax seed on beef quality and fatty acids in fattening cattle was studied in a single-factor design experiment. Thirty two Simmental crossbred beef with similar body weights were assigned to 4 groups with 8 duplicates in each group. The flax seed contents of the concentrates for group Ⅰ(control group), group Ⅱ, group Ⅲ and group Ⅳ were 0%, 5%, 10% and 15% respectively. 1)There were no significant differences in slaughter rate and pure meat percentage between group Ⅳ and the control group; flax seed significantly (P<0.05) increased back fat thickness and fat content of beef: the fat thickness of group Ⅳ increased 57.75% compared with the control group; 2)Adding flax seed to diet reduced shear force and pH value of longissimus at 24 and 48 h post-mortem (P<0.01); 3)Flax seed enhanced polyunsaturated fatty acid (PUFA) content of longissimus fatty acids significantly. These results indicate that increasing flax seed levels of diets can improve carcass quality. Flax seed improved the fatty acid profile, especially by increasing the amounts of PUFA. A comprehensive analysis showed that the optimal feeding level of flaxseed was 10%-15% of concentrate.