Aim To investigate effects of aging on acyl-CoA oxidase (ACO) expression in rat liver and eplore the molecular mechanism of lipid metabolism during aging. Methods Young (4～6 weeks) and aged (24 months) (16 per group) male SD rats were estimated the lipid level, and both of them were randomly divided into control and fenofibrate group (fed with fenofibrate for 2 weeks, and total dose was 0.5% of body weight). The level of liver ACO mRNA was measured by reverse transcription-polymerase chain reaction (RT-PCR). Results The triglyceride (TG) and total cholesterol (TC) levels of the aged control group increased significantly compared with the young control group. The ACO mRNA level of the aged group decreased significantly compared with the young group. Fenofibrate reduced serum TG and TC in aged- fenofibrate groups. The ACO mRNA level of both groups were affected by fenofibrate. Conclusion The mechanism of lipid dysfunction during aging is probably associated with the decreased expression of ACO.