The present invention relates to a composition for preventing or treating hyperlipidemia, fatty liver disease or arteriosclerosis, comprising an oxyntomodulin derivative as an active ingredient. The oxyntomodulin derivative has a high ability to activate GLP-1 receptor and glucagon receptor compared to native oxyntomodulin and has the effects of reducing the blood total cholesterol, low-density cholesterol and triglyceride levels that were increased by high-fat diet, and increasing high-density cholesterol levels and the high-density cholesterol/low-density cholesterol ratio. Thus, the oxyntomodulin derivative can be effectively used for the treatment of hyperlipidemia and related diseases.