Objective The efficacy and safety of add-on treatment of sitagliptin were assessed in patients with type 2 diabetes who were already on insulin therapy. Methods We started the add-on treatment of 50 mg of sitagliptin daily in 90 patients who were already on insulin therapy and with inadequate glycemic control. And insulin was titrated based on glycemic control to avoid hypoglycemia. After 16 weeks, the change in HbA1c level, daily dose of insulin, body weight and the incidence of symptomatic hypoglycemia were accessed in retrospective way. Results The patients were 65.8 years of mean age, and 23.8 of mean BMI with average duration of 12.7 years in Diabetes. The mean level of HbA1c in baseline was 7.52% and decreased to 6.78% at 16 weeks. The mean daily dose of insulin in baseline were 29.7 Units and 20.5 Units at 16 weeks. In 8 patients out of 90, insulin treatment became to be unnecessary. The level of BMI were stable and not increased, 23.8 in baseline and 23.4 at 16 weeks. Hypoglycemic events were less common. Conclusion Treatment with sitagliptin add to any kinds of insulin-based regimen were effective at lowering HbA1c and in decrease of daily insulin dose, and associated with less hypoglycemia and weight gain over 16 weeks.