Background: Knee Osteoarthritis (knee OA) is the most common joint disease in the elderly. It is a major cause of walking disability and balance impairment. The balance impairment or increase postural sway may result in increased risk of fall.Objectives: Compare effects of aquatic exercise and land-based exercise on postural sway and physical performance (pain, quality of life, leg muscle strength and leg muscle flexibility) in the elderly with knee OA.Subjects and methods: Fifty elderly women with knee OA were recruited for this study. They were randomized into aquatic exercise (n=25) and land-based exercise group (n=25). Postural sway views by anterior-posterior amplitude, medio-lateral amplitude, and total area were measured in subjects with eye opened or closed during double and single leg stance after six-week training, using the Force platform. Functional outcome was tested by the modified WOMAC. Knee injury and Osteoarthritis Outcome Score (KOOS), and pain scale were tested by Visual Analog Scale (VAS). Leg muscle strength was measured by chair stand, and leg muscle flexibility was measured by sit-andreach test. Aquatic group exercised at King Chulalongkorn Memorial Hospital therapeutic pool, while landbased group exercised at home.Results: In comparing postural sway before and after exercise, reduction in postural sway and pain, and increase in strength, the aquatic group were significantly better than those for land-based group (p <0.05). WOMAC, KOOS scores, and sit-and-reach test after six-weeks training were improved after exercise, but these were not significantly different between groups (p >0.05).Conclusion: Elderly patients with knee OA need exercise to improve balance and reduce sway. Six-week aquatic exercise was better than land-based exercise in sway reduction.