The purpose of this study was to explore the quality of life, and its predictors among middle-aged and elderly patients with Type 2 Diabetes Mellitus. 131 participants of age 40 or above were recruited from an outpatients department in one medical center in southern Taiwan. The study had a cross-sectional design; each participant was administered structured questionnaires, including the Diabetes Self-Care Behavior Scale, the Social Support Scale and the World Health Organization Quality of Life-BREF (WHOQOL-BREF). The results show that the predictors of patient's quality of life were diabetic self-care behaviors, economic status, and frequency of hospitalization, with a total variance of 21%. These findings can help health professionals to learn more about these patients. Furthermore, a suitable program and nursing interventions could be implemented to promote these patients' quality of life.