Knee osteoarthritis is one of the most common joint disorders of older individuals. It is a painful and disabling disease characterized by pain, stiffness. However, in Vietnam, the number of studies on this issue is still very limited, especially those on physical function. Objective: This study aimed to evaluate physical function in 178 elderly patients with knee OA. A cross-sectional study was utilized. Physical function was assessed by The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Timed up and go test (TUG). Mean total WOMAC scores of older patients with knee OA in the study was: 34.67 (± 15.63). The proportion of older patients with knee OA with impaired mobility (assessed by TUG test) was 64.8%. The mean total WOMAC scores were more likely to be higher in the elderly with knee OA whohave advanced age, living with others (family, caregiver), overweight/obese or underweight, poor nutritional status, no sleep disturbance, more pain, inflammatory pain and had no treatment for knee OA. There was a decrease in physical function in patients with osteoarthritis of the knee. Mobility impairment was associated with the level of pain, and treatment of knee OA or lack of.