Plasma cell neoplasms are generally categorized into four groups; multiple myeloma(MM), solitary plasmacytoma of the bone(SPB), plasma cell leukemias, and extramedullary plasmacytomas(EMP). These tumors may be further described as localized or diffuse in presentation. Localized plasma cell neoplasms are rare occurrences and include solitary plasmacytomas of the skeletal system, which account for 2-5% of all plasma cell neoplasms and extramedullary plasmacytomas of the soft tissue, which account for approximately 3% of all such neoplasms. A plasmacytoma is defined as any discrete, most likely solitary mass of neoplastic plasma cells either in the bone marrow or in various soft tissue sites. Diffuse lesions include the other two groups, multiple myeloma and plasma cell leukemia. The relationship between these processes has not yet been definitively characterized, but there appears to be a continuum in which both SPB and EMP often progress to MM. The patient was referred who had continuous deep throbbing bone pain and swelling on the left posterior gingival area of the mandible after extraction of the first and second molar. The result of intraoperative excisional biopsy of the lesion was confirmed as a plasmacytoma. And it revealed systemic multiple myeloma through the further diagnostic work-up. It is worth to report because of a rare case of multiple myeloma found in oral cavity as a form of plasmacytoma.