Original Article BACKGROUND: Nowadays, one of the most important methods for evaluation of treatment and care in diabetes mellitus, as a chronic disease, is to assess the death anxiety. The present study aimed to determine the relationship between cognitive fusion and cognitive distortion with death anxiety in patients with diabetes mellitus. METHODS: In this descriptive correlational study, the statistical population consisted of all patients with type 2 diabetes mellitus in Ardabil City, Iran, in year 2016. 110 patients with diabetes mellitus were selected as the statistical sample using the convenience sampling method. For data collection, the cognitive fusion scale, cognitive distortion scale, and death anxiety scale were used. Data analysis was made using Pearson correlation coefficient and multiple regression analysis at the significant level of P 0.050. RESULTS: There was a significant relationship between the cognitive fusion (0.59) and cognitive distortion (0.62) with death anxiety in patients with diabetes mellitus. Moreover, multiple regression analysis showed that cognitive fusion and cognitive distortion could predict the death anxiety among the patients with diabetes mellitus (0.48). CONCLUSION: It can be concluded that cognitive fusion and cognitive distortion are considered among the predicting variables related to death anxiety among the patients with diabetes mellitus.