[Purpose] The aim of this study was to investigate the relationship between clinical parameters, radiological staging and evaluated ultrasound results of quadriceps muscle thickness in knee osteoarthritis. [Subjects] The current study comprised 75 patients (51 female, 24 male) with a mean age of 57.9±5.2 years (range 40–65 years) and a diagnosis of osteoarthritis in both knees. [Methods] Knee radiographs were evaluated according to the Kellgren-Lawrence grading system. Clinical evaluation performed with the visual analog scale (VAS), Western Ontario and McMaster Osteoarthritis Index (WOMAC), the 50-meter walking test, and the 10-step stair test. The thickness of the muscle layer of the quadriceps femoris (M. vastus intermedius and M. rectus femoris) was measured with high-resolution real-time ultrasonography. [Results] The results of this study showed a significant negative correlation between quadriceps thickness and age, duration of disease, stage of knee OA, and VAS, WOMAC, 50-m walking test, and 10-step stair test scores. [Conclusion] The evaluation of quadriceps muscle thickness with ultrasound can be considered a practical and economical method in the diagnosis and follow-up of knee osteoarthritis.