The clinical significance of insulin and C-peptide measurement mainly includes diagnosis and typing of diabetes mellitus, judgment of pancreatic islet function, and judgment of the therapeutic effect of pancreatic islet cell tumor. 1. Diagnosis of diabetes mellitus: Type 1 diabetes mellitus patients’ pancreatic islet tissues are unable to secrete insulin normally, and the measurement of C-peptide will show a low and flat curve, whereas type 2 diabetes mellitus patients’ pancreatic islet function will be damaged to different degrees, and the measurement of C-peptide will show a lower-than-normal C-peptide concentration, which will lead to a clear diagnosis. 2. Determination of islet function: C-peptide is not affected by the injection of insulin, and the change of C-peptide can reflect the function of pancreatic islet B-cells in patients who receive insulin treatment. When there are abnormal changes in pancreatic islet function, it is necessary to consult a doctor for C-peptide examination in time, which is very helpful for the diagnosis and treatment of the disease.