Knee osteoarthritis prevalence is 60 to 90 percent among 65-year-old and older people, and it is one of the most important causes of pain and disability in the elderly, on which muscle weakness as a risk factor plays an important role. So far, muscle weakness and atrophy in patients with osteoarthritis was attributed to lack of muscle function, and it thought that patients have less mobility due to joint pain, and those joint muscles are used less. The aim of this study was to investigate the effect of eight weeks of strength training in the water on pain, a range of motion, and muscle strength in postmenopausal women with knee arthritis. A total of 30 postmenopausal women with knee osteoarthritis were purposefully selected and randomly divided into two groups of 15 individuals of strength training in water and control (no training) groups. The degree of pain was assessed with the use of VAS questionnaire, the range of motion with the use of the goniometer, and the muscle strength with the use of a dynamometer. The experimental group performed the strength training in water for eight weeks, three sessions of 1 hour in each week. The paired t-test was used for comparison within a group and the independent t-test was used for comparison between groups (P<0.05). The pain (P=0.001) reduced about 35%, the range of motion increased about 21% (P=0.001), and the quadriceps muscle strength (P=0.004) increased about 70%, which significantly improved in the exercise group compared to the control group (P≤0.05). The strength training program in water had a positive effect on improving pain, the range of motion, and quadriceps muscle strength in postmenopausal women with knee osteoarthritis, and it could be recommended as a safe and effective treatment for them.