De Quervain's disease is the most common form of tenosynovitis, which causes disability in daily living and occupational activity. Anatomical variations in the first extensor compartment including separate comparment influenced the effects of treatment. A 45-year-old woman had severe pain on wrist and suffered in activity of daily living after motor vehicle accident. She had swelling and tenderness upon the radial styloid process, and more aggravated pain by Finkelstein's maneuver test. The magnetic resonance imaging (MRI) scans showed severe peritendinous edema within the synovial sheath, increased signal intensity within the tendons. Also, the images revealed a thickened septum between abductor pollicis longus and extensor pollicis brevis. Corticosteroid was injected on both sides of the septum. We report a case of definite septum of de Quervain's disease via MRI scans.