Objective: To investgate the electromyographic and pathological features of peripheral nerve involvement in MS, and clarify factors that affect the involvement of PNS. Methods: Thirty-three multiple sclerosis (MS) patients were selected according to Poser's definite diagnosis of MS, without other nervous system disorders and thirty volunteers served as controls. All subjects received nerve conduction study: MCV, SCV, F-wave and H-reflex. Sural nerve biopsy and pathological change were observed in light and electronic microscope. Results: Electromyograph findings: decreased CMAP of the median, tibial and peroneal nerves, and high amplitude of SAP of median and ulnar nerves were observed in MS. Our study also showed the prolonged F wave response and H reflex in median and tibial nerves, low F wave rates. Electrophysiologic abnormalities in MS showed close relationship to the EDSS, duration and lesion position in spinal cord. Sural nerve biopsy revealed myelinated nerve fibers with segmental demyelination by light microscope. Inflammatory cellular infiltration and onion-bulb formation were not found. Axonal degeneration and laminae dissociation in myelin sheath were demonstrated by electromicroscope. Conclusion: MS is a demyelinating disease attacking to CNS. In some MS patients, both CNS and PNS might be involved at the same time. The peripheral demyelinating lesions seem to be common in spinal nerve root. But the secondary axonal degeneration in distal segment should be another pathologic feature of MS. EMG is a useful test method to evaluate the peripheral nerve damage and prognosis of MS.