The objective of the current study was to evaluate the effects of rumen undegradable unsaturated fat (RUF) on growth performance, biochemical indices and related gene expression of Angus beef. Twenty-four Angus steers [average body weight of (447.78±2.53) kg, 15 to 17 months of age] were randomly assigned to 4 groups with 6 beef in each group. Groups 1, 2, 3 and 4 were fed experimental diets containing 0, 2.0%, 4.0% and 6.0% RUF, respectively. The experiment lasted 77 days including a 10-day preliminary test period and a 67-day formal test period, and the formal test period was divided into two stages; early stage (27 days) and later stage (40 days). The results showed as follows: 1) in early stage, the average daily gain of group 4 was significantly higher than that of groups 1 and 2(P < 0.05) the ratio of feed to gain of group 1 was signifi- cantly higher than that of group 4 (P < 0.05) . In early stage, later stage and whole stage, there was no signifi- cant difference on the average daily feed intake among all groups P > 0.05). 2) In early stage and later stage, the serum insulin (INS) content of groups 2 and 3 was significantly higher than that of group 1 (P < 0.05), the serum glucagon (GC) content of groups 2, 3 and 4 was significantly higher than that of group 1 (P < 0.05) . In early stage, the serum growth hormone (GH) content of group 3 was significantly lower than that of group 1 (P < 0.05) in later stage, the serum GH content of groups 2, 3 and 4 was significantly lower than that of group 1 (P<0.05). 3) In early stage and later stage, there were no significant differences on the activities of adipose triglyceride lipase (ATGL), carnitine palmitoyl transferase- I (CPT-I) and acetyl-coenzyme A car- boxylase (AACase) in serum among all groups (P > 0.05) 4) In early stage and later stage, there were no significant differences on the glucose (GLU), triglyceride (TG), albumin (ALB) and total cholesterol (TCHO) contents and alanine aminotransferase (ALT) activity in serum among all groups P > 0.05) . In ear- ly stage, the serum total protein (TP) content of group 4 was significantly higher than that of group 1 (P< 0.05), the serum malondialdehyde (MDA) content of groups 2, 3 and 4 was significantly lower than that of group 1 (P < 0.05) the serum glutamic oxaloacetic transaminase (AST) activity of groups 2, 3 and 4 was significantly higher than that of group I(P < 0.05); in later stage, the serum (MDA content of groups 3 and 4 was significantly lower than that of group 1(P < 0.05) the serum AST activity of groups 2 and 3 was signifi- cantly higher than that of group I(P < 0.05) * 0.5 The gene expression of peroxisome proliferator-activated re- ceptor y (PPARy) in Longissimus dorsi of group 4 was significantly higher than that of group 1 (P < 0.05) . In conclusion, dietary RUF has no significant effects on the average daily gain, average daily feed intake and ratio of feed to gain of beef in whole stage, and has no significant effects on the activities of aATGL, CPT-I and AACase in serum, but improves the contents of GC and INS in serum, promotes the gene expression of PPARY in Longissimus dorsi. Overall consideration, under this experimental conditions, the optimum dietary RUF content is 4.0%.