BACKGROUND: C-peptide is a useful marker to differentiate the cause of hypoglycemia and assessment of beta cell secretory capacity in diabetes mellitus. On Modular E170 (E170, Roche Diagnostics, US), an immunoassay autoanalyzer based on the electrochemiluminescence technology, we evaluated the performances of C-peptide measurement and established the normal reference range whether suitable to replace the conventional radioimmunoassay (RIA). METHODS: Control materials and patient sera were used for evaluation of precision and linearity and for comparison with RIA method. Reference range was determined with 130 adults chosen as a reference population. All procedures were done according to NCCLS guidelines. RESULTS: Within-run and total precision of C-peptide measurement in E170 with five different control materials were all below 2 %. The linearity was excellent up to 60.4 ng/mL. The Passing-Bablok linear regression analysis showed excellent correlation with conventional RIA method (R=0.9862). The reference range determined from a reference population was 0.75-2.30 ng/mL. CONCLUSIONS: C-peptide measurement with E170 had satisfactory performance and the results had excellent correlation with RIA results. Adopting new automated electrochemiluminescence immunoassay would improve the overall performance of laboratory in terms of precision, turnaround time and overcoming the drawbacks of a conventional RIA.