A composition for preventing or treating diabetes mellitus includes insulin and a GLP-1/glucagon dual agonist. The composition can inhibit the weight gain and reduce the danger of hypoglycemia due to the administration of insulin, lower the administration dose and greatly improve the compliance of drugs through a combined administration of a long-acting insulin conjugate and a long-acting GLP-1/glucagon dual agonist conjugate. In addition, the long-acting insulin conjugate and the long-acting GLP-1/glucagon dual agonist conjugate can improve the in vivo sustainability and stability because an insulin and a GLP-1/glucagon dual agonist are linked to the immunoglobulin Fc region via a non-peptidyl linker. A method for preventing or treat diabetes mellitus includes administration of the insulin and a GLP-1/glucagon dual agonist.