Abstract : The effect of positive thinking on the level of depression in diabetic Mellitus patients. Diabetes mellitus is a chronic disease that cannot be cured. It is not contagious but has a high risk of disrupting one's life. This study aims to prove the effect of positive thinking on the level of depression in patients with diabetes mellitus. This type of research is a pre-experimental One-Group Pre-test-posttest Design. The sampling technique is purposive sampling. The number of samples in this study was 34 people. Data collection techniques used Beck Depression Inventory-II (BDI-II). The results of the investigation before treatment were positive thinking depression level in study subjects, mild depression in 12 people (35.3%), depression was 18 (52.29%), and significant depression for 4 people (11.8%). After being given a favorable thinking treatment, the depression level of the study subjects was: normal/not depressed in 1 person (2.9%), mild depression in 17 people (50%), moderate depression in 15 people (44.1%), and significant depression 1 person (2.9%). Paired t-test results: mean before treatment 21.9412, after treatment 19.1765, t = 5.787, and p = 0.000. There is a significant effect of positive thinking on the level of depression in patients with diabetes mellitus. Positive thinking can reduce depression levels in patients with diabetes mellitus.