Background & Aims: KNEE instability has become a debilitating problem among patients with KNEE osteoarthritis which may have many causes and complications. In this study, researchers evaluated the KNEE Flexion Contracture (KFC), function, quality of life, and balance and its relationship with KNEE instability (KI). In this study, the researchers investigated the function, quality of life, KNEE Flexion Contracture (KFC), and balance of the patients with KNEE osteoarthritis and its relationship with KNEE joint instability. Materials & Methods: In this double-blinded case-control study, 60 females with KNEE osteoarthritis participated and divided into two groups (30 in each) of osteoarthritis (mean age: 52. 80) and without KNEE instability (mean age: 54. 43) based on the KNEE instability index scores. Pain was evaluated with a visual analog scale(VAS), KFC with a universal goniometer, performance with WOMAC, and physical performance of the patients with the time up and go (TUG) and and time of up and down stairs tests, quality of life with the SF36 questionnaire, and balance with the Single Leg Lift test. Data were analyzed by SPSS version 22 software and independent sample t-test at a significance level of 0. 05. Results: The mean scores of KFC showed significantly difference between the two groups with and without KI (right foot: P=0. 0001, left foot: P=0. 001). Also, the mean scores of pain, fall times, WOMAC, physical function, quality of life, and One-legged balance were significantly different between groups (P=0. 0001 in all variables). Regarding the time of up and down stairs, there was no significant difference between the two groups (P=0. 057), although the difference between them was significant in the descent time (P=0. 0001). Conclusion: According to the findings of this study, it was found that there was a significant difference between the two groups of osteoarthritis with and without KI in pain, function, KFC, quality of life, and One-legged balance, which paying attention to the causes and side effects of this problem makes it more clear.