Abstract： Objective To explore the diagnostic value of serum C-peptide and glycosylated hemoglobin A1c in diabe-tes mellitus.Methods Two hundred patients with suspected diabetes mellitus admitted to Xinglin Branch of the First Affiliated Hospital of Xiamen University from January 2021 to December 2023 were selected as the study objects.The levels of C-peptide and glycated hemoglobin A1c were detected in all patients.Glucose tolerance test was used as the gold standard to analyze the combined diagnostic value of the two indexes.Results The results of glucose tolerance test showed that 165 of the 200 suspected patients were diagnosed with diabetes mellitus and 35 were non-diabetic.The detection rate of serum C peptide combined with glycated hemoglobin A1c was 83.50%,higher than that of single index of 72.50%and 74.50%.The accuracy sensitivity and negative predictive value of combined diagnosis of diabe-tes were higher than those of single index,and the differences were statistically significant(all P<0.05).Conclusion Serum C-peptide in combination with glycated haemoglobin A1c may be of definite value in the diagnosis of diabetes.