Arthritis is one of the most common chronic degenerative joint disease in elderly. Osteoarthritis is a widespread, slowly developing disease, with a high prevalence increasing with age in women. The large joints mostly involved by the disease are the knees. But there are no treatments available that cure the underlying process of osteoarthritis diseases. Physical exercise helps in increasing cartilage nutrition and remodeling, increases the synovial blood flow, decrease swelling, and improves muscle strength. Thus, exercise has been suggested as an important nursing strategy in osteoarthritis. Purpose: The purpose of this study were to compare muscle strength between Tai-Chi exercise and aquatic exercise for women with knee osteoarthritis. Methods: A quasi-experimental study with pretest and posttest measures was used. The study subjects were those who had been enrolled in a community health center, and agreed to participate in the study for eight weeks, signed the consent form, and obtained the physicians approval. The study dropout rates were 13.2% with the final study subjects of 17 on Tai-Chi exercise, 16 on aquatic exercise program. The collected data were analyzed using SPSS for Window (version 12.0). Independent sample t-test and paired t-test was performed to compare of muscle strength for women with osteoarthritis after 8-week Tai-Chi exercise and aquatic Exercise. Results: The homogeneity tests of demographic characteristics and study variables at the pretest data revealed no significant differences between two groups. After 8-week Tai-Chi and aquatic exercise, there was significant result in pre-post test comparison on muscle strength on Tai-Chi group, but no significant in aquatic group. There were no significant differences of knee extensor (p=.078), and hand grip(p=.118) in group comparisons on muscle strengths. But there were significant differences of knee flexor(p=.024). Conclusion: Tai-chi exercise was effective in improving knee flexor. So, it seems that Tai-chi exercise may be more suitable for aquatic exercise in osteoarthritis exercise programs. Further studies with other comparisons in physical and psycho social outcomes are necessary to confirm the more effects of exercise.