Abstract: Type 2 diabetes is a complex metabolic disease, accompanied by insulin resistance and elevated blood glucose. As the disease progresses, hyperglucagonemia will occur. Glucagon has a significant effect on glucose increase and energy expenditure. In recent years, several glucagon receptor (GCGR) antagonists were developed. They lowered blood glucose in clinical studies, along with side effects, such as increased blood lipids and elevated liver transaminase. In order to solve these problems, glucagon like peptide 1 receptor (GLP-1R)/GCGR co-agonists were developed, which not only lower blood glucose, but also reduce weight and promote lipolysis. In this review, we will focus on the biological effects of glucagon, the treatments of GCGR antagonists, and GLP-1R/GCGR co-agonists on type 2 diabetes.