Purpose: The purpose of this study was to examine the effect of Tai Chi exercise on muscle strength, grip strength, flexibility, pain, depression and self-efficacy in patients with osteoarthritis. Method: With a quasi-experimental design, 46 persons with osteoarthritis were assigned into an experimental group (n=22) or a control group (n=24). Muscle strength, grip strength, flexibility, pain, depression and self-efficacy were measured both before and after the exercise. Data were analyzed with SPSS WIN using descriptive statistics, t-test, and ANCOVA. Results: The mean age of the subjects was 68 years in the experimental group and 64 years in the control group. After controlling for age and pre-test outcome variables, muscle strength scores (F=-14.8, p= .00), grip strength scores (Rt: F=8.40, p= .01), and self-efficacy (F=47.3, p= .00) improved more in the experimental group than the control group. Joint pain scores (F=15.0, p= .00) and depression scores (F=14.7, p= .00) decreased in the experimental group. Conclusion: Tai Chi exercise is likely to be an effective nursing intervention to improve muscle strength, grip strength, and self-efficacy and reduce pain and depression for patients with osteoarthritis.