Objective: The goal of the present study was to evaluate the efficacy of sitagliptin when added to insulin therapy in patients with type 2 diabetes in a real clinical practice. Methods: A total of 100 mg sitagliptin was added once daily to basal insulin therapy with oral glycemic agents (group 1:metformin, group 2:metformin + sulphonylurea). We evaluated the HbA1C change from baseline at week 12. The study assessed clinical factors that affect the efficacy of sitagliptin treatment after 12 weeks. Results: In the group taking metformin (n = 30), treatment with sitagliptin for 12 weeks significantly decreased HbA1C compared with baseline (mean HbA1c change -0.7%). In the group taking metformin with sulphonylurea (n= 31), treatment with sitagliptin for 12 weeks also significantly decreased HbA1C compared with baseline (mean HbA1c change -0.6%). Logistic regression analysis showed that a reduction in HbA1c levels after 12weeks was significantly associated with higher baseline HbA1c (P = 0.029). Conclusion: The efficacy of sitagliptin when added to insulin therapy in type 2 diabetic patients appears to be effective in Korean patient with type 2 diabetes who showed inadequate glycemic control.