Purpose. Tai Chi exercise, an ancient Chinese martial art, has drawn more and more attention for its health benefits. The purpose of the study was to identify the effects of a Sun-style Tai Chi exercise on arthritic symptoms (joint pain and stiffness), motivation for performing health behaviors, and the performance of health behaviors among older women with osteoarthritis. Methods. Total of 72 women with the mean age of 63 years old were recruited from outpatients clinic or public health centers according to the inclusion criteria and assigned randomly to either the Tai Chi exercise group or the control. A Sun-style Tai Chi exercise has been provided three times a week for the first two weeks, and then once a week for another 10 weeks. In 12 weeks of study period, 22 subjects in the Tai Chi exercise group and 21 subjects in the control group completed the posttest measure with the dropout rate of 41 %. Outcome variables included arthritic symptoms measured by K-WOMAC, motivation for health behavior, and health behaviors. Results. At the completion of the 12 week Tai Chi exercise, the Tai Chi group perceived significantly less joint pain (t = -2.19, P = 0.03) and stiffness (t = -2.24, P = 0.03), perceived more health benefits (t = 2.67, P = 0.01), and performed better health behaviors (t = 2.35, P = 0.02), specifically for diet behavior (t = 2.06, P = 0.04) and stress management (t = 2.97, P = 0.005). Conclusion. A Sun-style Tai Chi exercise was found as beneficial for women with osteoarthritis to reduce their perceived arthritic symptoms, improve their perception of health benefits to perform better health behaviors.