Rett syndrome is a progressive encephalopathy in females that appears during the first 18 months of the life. A few neurophysiologic investigations of peripheral nerve and electrodiagnostic studies in Rett syndrome has proposed mild distal axonopathy. However, the electrodiagnostic study shows demyelination rather than axonopathy in presenting patient with Rett syndrome. An 11-year-old female had normal birth history, no perinatal problems and normal development until age of 18 months. Developmental regression was recognized by her parent and slowly progressed. At physical examination at her age of 7 years, stereotypic hand movements was present and she could walk with frequent fall. Brain Computed Tomography was not specific and electrodiagnostic study revealed slow conduction velocity in all nerves tested. After follow-up for 4 years, hand wringling was remained and she became to be unable to walk alone. Electrodiagnostic study revealed as follows; 1) Compound muscle action potentials and sensory nerve action potentials in all the nerves tested revealed prolonged distal latency with normal amplitude. 2) F waves were evoked with prolonged latency in the all the nerves tested. 3) H-reflex were absent, bilaterally. 4) Facial motor conduction study and blink reflex showed normal findings. 5) Auditory evoked potential and visual evoked potential studies showed normal findings. These electrodiagnostic study indicates demyelinating neuropathy.