Objectives: Comparing the effect of Tai chi and close kinetic chain exercise on the knee osteoarthritis. Design: double-blind study. The patients will be treated in three groups: close kinetic chain exercises, Tai chi and control group. Patients will be assessed before treatment, after the sixth, twelfth and one month after treatment. Methods: subjects are men and women with grades 2 and 3of knee osteoarthritis, age 45-65 years, referring to the Municipality clinic in region 13 of Tehran. Sample size: 16 people in each group. Interventions: Applying ultrasound (1MHZ, 80 % pulse, intensity 1W/Cm2, 5 min), infrared ( 15 minutes) in three groups. Tai chi group: 5 minute warm up, 5 minute cool down exercises at the beginning and the end of sessions, 10 minutes of the 6th. Form of Yang style of tai chi exercise. Close kinetic chain exercise group: 5 minute static stretching as warm up at the beginning and 5 minutes static stretching as cool down at the end of sessions, 10 minutes exercise including standing terminal knee extension, mini squat in 15 degree of knee flex, step up from the front and side and lunge exercises. Main outcome measures: measuring of pain by visual Analog scale, flexion and extension knee range of motion by goniometry, joint swelling above and 5 cm upper and lower of the patella by tape, performance by the KOOS questionnaire and 6 -minute walk test, balance by Berg balance scale.