Background: Osteoarthritis is an inflammatory condition of a synovial joint in which the cartilage is damaged over time until pressure between the bones causes pain. The aim of this study aimed to evaluate the effect of health behavior modification based on trans theoretical model among osteoarthritis patients. Research design: A quasi experimental design was utilized in this study. Setting: This study was conducted at Orthopedic Outpatient Clinic affiliated at Benha University Hospital in Benha City, Egypt. The sample: A purposive sample used in this study involved 112 patients with osteoarthritis. Tools: Two tools were used I): A structured interviewing questionnaire which consists of three parts 1): Demographic characteristics and medical history, 2): Patients' knowledge regarding osteoarthritis and trans theoretical model and 3): Patients' behaviors II): Questionnaire of the trans theoretical model of behavior modification through the stages of modified change. Results: 81.3% of the studied patients had good knowledge while 72.8% accepted healthy behaviors and 79.5% of the studied patients applied trans theoretical model. There were statistically positive correlations between behaviors and trans theoretical model pre and post application. Conclusion: The trans theoretical model succeeded to improve behaviors of the studied patients. Recommendations: Further studies are recommended to explore the effect of trans theoretical model on behavior modification through stages of change.