Background: Diabetes mellitus continues to be a global health problem with increasing importance across the world by affecting the activities of daily living and self-care ability of patients due to its incidence and troubles caused by it. The present study aims at determining the effect of activities of daily living of patients with type 2 diabetes mellitus on their self-care agency. Methods: The population of this descriptive study consisted of patients with type 2 diabetes mellitus who were being treated at the internal medicine clinic of a provincial state hospital between July 2014 and November 2015 and its sample consisted of 150 diabetic patients who volunteered to take part in the study and who were open to communication. A personal information form, the Activities of Daily Living (ADL) Scale, the Instrumental Activities of Daily Living (IADL) Scale and the Self-Care Ability Scale (CAS) were used as data collecting tools. The data were analyzed using descriptive statistics (numbers, percentages, mean, standard deviation, mean rank and frequencies), the Shapiro-Wilk, KruskalWallis Variance Analysis, Mann-Whitney U test and Cronbach's alpha formula and correlation analysis. Results: Approximately 97.3% of the patients with type 2 diabetes stated that they were independent in ADL and 75.3% of them in IADL. The patients' mean self-care ability score was found to be 83.85 ± 17.87 ADL and IADL were found to be affected by age, marital status, education, duration of disease, willingness to receive further education, and presence of another disease besides diabetes. There was a significant correlation between the self-care agency score and marital status, education, duration of disease, willingness to receive further education, presence of another disease besides diabetes, regular checking of blood sugar and compliance with diet. A significant positive correlation was found between the patients' activities of daily living and their self-care agency. Conclusion: The patients' activities of daily living were found to affect their self-care agency. The personal and disease-related characteristics of patients should be identified so that their self-care behaviors can be increased.