Introduction Diabetes mellitus (DM) is one of the major public health problems with a significant chronic medical, social, and financial impact and has come to be considered a global health emergency of the 21st century., The health belief model gives ways to have the knowledge of the attitude, health-related behaviors, and peoples' educational needs and therefore, can be used as an applied tool to form impressive public health intervention strategies. Aim of the study: This study aimed to assess the effect of applying Health Belief Model (HBM) on type 2 diabetic patients' knowledge, self-care practices, and health beliefs. Study hypothesis: Application of HBM will improve diabetic patients' knowledge, self-care practices, and health beliefs and in this way will control type 2 diabetes mellitus. Methods: Study design: A quasi- experimental, one-group, (pre and post-test) study design was used. Setting: This study was conducted at the diabetic outpatient clinics of the Medical Specialized Hospitals/Mansoura University, Dakahlia Governorate, Egypt. Subjects: A purposive sample of 72 type 2 diabetic patients who agreed to participate in the study was utilized. Results: 1.The study results revealed highly statistically significant differences with an improvement in total patients’ knowledge and self-care practices’ score in post-educational intervention based on HBM as compared with pre-intervention. 2.post-intervention, the mean score of each HBM constructs: susceptibility, severity, benefits, cues to action, and self-efficacy were increased significantly (except the barriers construct which was declined) compared to pre-intervention (P-value <0.000). Conclusion: The application of the HBM model was proved to be effective in increasing knowledge, promoting the self-care practices and health beliefs of patients with type 2 diabetes mellitus. Recommendation: Increasing people’s awareness of their disease, treatment adherence, adopting a healthy lifestyle via education and training classes should be emphasized. Besides, supporting, continuous, and well-designed training programs for healthcare professionals should be established to apply HBM on chronic disease prevention and control.