We aimed to evaluate the ability of serum C-peptide levels to predict glycemic control in obese type 2 diabetes (T2DM) patients in which a glucagon-like peptide-1 receptor agonist (GLP1-RA), an exenatide, was added to metformin monotherapy. This was a retrospective study, in which we enrolled 44 consecutives obese, type 2 diabetic patients receiving metformin monotherapy and have inadequate glycemic control (HbA1c >7% and <10%). Twice daily GLP1-RA (10 mcg exenatide injection) was added to the treatment. Weight, height and body mass index (BMI), in addition to fasting plasma glucose, hemoglobin A1c (HbA1c), and C-peptide levels measured baseline and at the sixth months post treatment. Regardless of the initial HbA1c level, treatment success was considered a HbA1c level below 7%. Predictors of successful glycemic control were assessed by a regression analysis. The fasting plasma C-peptide level was used as a marker of β cell function. After adding the GLP-1 RA, fasting glucose, C-peptide, BMI, and body weight decreased significantly (p < 0.01 for all). 27 (61.4%) patients were achieved treatment success, who have HbA1c level < 7% at the sixth month. The baseline C-peptide level was correlated with the HbA1c level 6 months post-treatment (r: 0.4, p: 0.01). Multivariate logistic regression analysis showed that the baseline fasting plasma C-peptide level was an independent predictor of successful glycemic control [exp.B: 6.6 (1.63-26-9) p: 0.008]. In a receiver operating characteristics (ROC) curve analysis, a baseline plasma C-peptide level of 2.56 ng/mL was the best cut-off value. Initial fasting plasma C-peptide levels can predict the treatment response of the GLP1-RA (exenatide 10 mcg, twice daily) add on to metformin monotherapy in obese type 2 diabetics.