Context: Osteoarthritis is an intractable joint disease that is characterized by chronic pain, rigidity and functional disability with significant costs and a great impact on health and quality of life. In order to mitigate osteoarthritis patients' disability and complaints, they must adhere to optimal self-care. Aim: To evaluate the effect of self-care program on pain intensity and functional ability of osteoarthritis patients. Method: The study used a quasi- experimental design, specifically employing a pre/post-test methodology on a purposive sample of (80) patient with osteoarthritis admitted within four months to orthopedic department and orthopedic outpatient clinics at Benha University Hospital, Qualubyia Governorate, Egypt. Tools: Four tools were utilized for data collection: Structured interview questionnaire, osteoarthritis self-care practice scale, numeric pain rating scale and functional ability of osteoarthritis patients (Katz activities of daily living and the lawton instrumental activities of daily living scale). Results showed a statistically significant increase in the total patients' knowledge level both immediately and post 2 months of self-care program implementation (P ≤0.05). There was a statistically significant enhancement in the total self-care practice mean score, whereas preprogram the total mean score was 21.33 ± 7.09 indicating unsatisfactory level of practice which increased immediate and post 2 months of self-care program implementation to 37.13 ± 6.25 and 36.03 ± 6.27 respectively indicating satisfactory level of practice (P<0.001). Besides, a statistically significant difference in the total mean score of patients' functional ability according to their instrumental   activities   of   daily living pre and post 2 months of self-care program implementation (p= <0.001). Conclusion: The implementation of a self-care program resulted in a significant and positive effect on the functional ability of patients and the reduced intensity of osteoarthritis pain. Recommendations: Further researches and studies are needed to focus on identifying new approaches of self-care practice to alleviate osteoarthritis pain intensity and enhance functional ability.