Cardiovascular diseases (CVDs) are the most common cause of mortality and morbidity globally. This is due to the increasing prevalence of modifiable CVDs risk factors. Hence, the study was aimed to identify knowledge and unhealthy behaviors that contribute to CVD among diabetes mellitus patients. An institutional-based cross-sectional study design was employed among diabetes mellitus patients. A close-ended questionnaire developed from up-to-date similar literature was pretested and face-to-face interview techniques were used to collect data. Checked data were entered into the Epidata 4.1 versions. Then, descriptive and bivariate logistic regression was done using SPSS 25 versions. Of the study participants included in the study, 318, more than half of them, 167(52.5%), were aged ≥45 years and 187(58.8%) of them were females. Among the study participants, more than half, 198(62.3%), had good Knowledge of modifiable CVDs risk factors. Most of the study participants identified consuming foods rich in fats instead of vegetables and fruits 198(62.3%), followed by physical inactivity 196(61.6%) as a risk factor for CVD. Regarding CVDs prevention practice, 175(55.0%) of the patients had a good practice. More than three-fourths, 267(84.0%), practice avoiding foods rich in fats and, sugar, and cigarette smoking 250(78.6%). Predictors like educational status, college and above (AOR 2.68; 95% CI 1.14–6.27), and urban residence area (AOR 1.94; 95% CI 1.09–3.15) were associated with knowledge of CVDs risk factors. While sex, marital status monthly income, and age of the participants had no association with knowledge and prevention practice of modifiable CVDs risk factors. The study participants’ knowledge and prevention practice of modifiable cardiovascular disease risk factors was satisfactory, even though continuous awareness creation is required to lower CVD mortality and morbidity burdens. Educational status and residence are of the study participants affect the knowledge and prevention practice modifiable of cardiovascular disease risk factors.