AIMS To examine the association of connecting peptide (C-peptide) and the risks of postpartum diabetes and pre-diabetes among women with prior gestational diabetes. METHODS A cross-sectional study of 1263 women with prior gestational diabetes was carried out at 1-5years after delivery in Tianjin, China. Logistic regression was used to assess the associations of C-peptide and the risks of diabetes and pre-diabetes. RESULTS The multivariable-adjusted odds ratios based on different levels of C-peptide (0-33%, 34-66%, 67-90%, and >90% as C-peptide cutpoints) were 1.00, 1.93 (95% confidence interval [CI] 0.85-4.39), 2.49 (95% CI 1.06-5.87), and 3.88 (95% CI 1.35-11.1) for diabetes (P for trend <0.0001), and 1.00, 1.66 (95% CI 1.18-2.36), 2.38 (95% CI 1.56-3.62) and 2.35 (95% CI 1.27-4.37) for pre-diabetes (P for trend <0.0001), respectively. Restricted cubic splines models showed a positive linear association of C-peptide as a continuous variable with the risks of type 2 diabetes and pre-diabetes. The positive association was significant when stratified by healthy weight and overweight participants. CONCLUSIONS We found a positive association between serum C-peptide levels and the risks of diabetes and pre-diabetes among Chinese women with prior gestational diabetes. Our finding suggested that elevated C-peptide levels may be a predictor of diabetes and pre-diabetes.