Background: Osteoarthritis is known as degenerative joint disease and typically the result of wear and tear and progressive loss of articular cartilage. The aim of this study aimed to evaluate the effect of trans theoretical model on knowledge and practices 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 to assess 1): Demographic characteristics and medical history, 2): Patients' knowledge regarding osteoarthritis and trans theoretical model and 3): Patients reported practices II): Questionnaire of the trans theoretical model through the stages of modified change. Results: 81.3% of the studied patients had good knowledge while 71.4% of the studied patients had satisfactory practices, and 79.5% of the studied patients applied trans theoretical model. There were statistically positive correlations between knowledge, practices, and trans theoretical model pre and post application. Conclusion: The trans theoretical model succeeded to improve knowledge and practices of the studied patients. Recommendations: Continuous application of trans theoretical model for patients with osteoarthritis to enhance patients` knowledge and practices.