Background: Quality of life is an illustration of an individual’s livelihood, as related to daily activities, in order to own a typical life, especially with reference to health. Individuals with diabetes mellitus have a quality of life that is quite diverse and affects the health conditions of all related people. It is this point that the role of locus of control is contemplated in maintaining the quality of life for these individuals. This study aim to analyze the influence of locus of control in patients with diabetes mellitus, in relation to the quality of life they lead. Methods: This observational survey based study was performed by implementing a cross-sectional design. The study was conducted in type B and C hospitals and health centers of Makassar City and included 389 respondents. Data analysis was done using chi-square test. Result: The result showed that the locus of control had a significant effect on the quality of life of patients with type 2 diabetes mellitus (ρ<0.01; α=0.05). Conclusions:. People with type 2 diabetes mellitus, who had an external locus of control, were observed to be more dominant and did not own a good quality of life because such individuals are very dependent on the external environment rather than the environment within them. The result of this study also indicate that locus of control may be regarded as the basis in predicting the future quality of life of an individual.