13 patients with extra-medullary plasma cell tumor treated by radiotherapy in a Cancer Hospital from March 1958 to December 1983 are reported. The tumor occurred more frequently in the head and neck, particularly in the upper respiratory tract and oral cavity. Grossly, the majority of the tumor showed dark reddish polypoid masses with the surface bled easily. A misdiagnosis of polyp or hemangioma was usually made. The tumor is sensitive to radiotherapy. In the patients with recurrence or metastasis, retreatment is still effective, at least, palliative. The 5 year survival rate was 70%. A dose of 4,000-6,000 rad in 4-6 weeks is advised for the primary tumor.