Objectives : This study aims to estimate the oral antihyperglycemic medications adherence among ambulatory care with adult type 2 diabetes patients and to identify factors affecting the medication adherence in Korea. Methods : This study used the Korean National Health Insurance Database. Study population was 40,082 patients who were 20 years of age or older and first diagnosed with type 2 diabetes (ICD-10: E11) in 2004. The patients were followed up for two years in order to measure adherence with oral antihyperglycemic medications. The level of medication adherence was measured by the medication possession ratio (MPR). Results : The average MPR in the study population was 49.5%. The appropriate adherence rate (MPR$geq$80%) was 29.4% and showed variation according to the characteristics of individual patients. Multiple logistic regression analysis revealed that the odds of appropriate adherence increased with female (OR:1.21, CI:1.14-1.27), older age, increasing ambulatory care visits, health insurance (OR:1.53, CI:1.33-1.76), decreasing ambulatory care providers, using a specialized general hospital as their main attending medical institution (OR:10.08, CI:8.96-11.33), having co-morbidity, using polytherapy (OR:1.07, CI:1.01-1.13). Conclusions : The medications adherence for patient with type 2 diabetes is low in Korea, and shows variation according to the characteristics of patients. For proper management of diabetes, health care policy is expected to be enacted to improve medications adherence continuously. In particular, more intensive management is needed for patients with low medications adherence. Also, health care policy makers need to develop the program to induce health care utilization by a patient to be more concentrated with the same provider.