Diffuse large B-cell lymphoma (DLBCL) is the most common non-Hodgkin's lymphoma, and usually showed painless neck swelling, fever, sweat, and weight loss. Although about 5% of all lymphomas appeared in the oral area, the primary maxillofacial lymphomas were rare and sometimes clinically tended to be misdiagnosed such as chronic periodontitis, osteomyelitits, etc. This study demonstrated three cases of primary DLBCL mimicking localized osteomyelitis at mandible or maxilla. A series of histological and immunohistochemical examination using different biomarkers of lymphoreticular cells were performed to characterize the neoplastic cells of DLBCL. The first case occurred in a 45 years old male exhibiting mandibular osteomyelitis and neck swelling. The second case simply showed a gingival swelling at left upper premolar area in a 55 years old male. And the third case is from an 84 years old female who felt numbness at left lower lip and had severe periodontitis involving regional alveolar bone resorption. All of three cases had experienced no systemic manifestation of lymphoreticular malignancy before the diagnosis of oral lymphoma. Immunostainings of CD3, CD20, TNFα, BCL-2, Ki-67, PCNA, and c-Myc were strongly positive in these tumor cells, while those of p53 and CD31 were slightly positive, and CD56 immunoreaction was negative. These three cases were diagnosed as DLBCL and referred to the hemato-oncology unit for treatment. Therefore, every chronic granulomatous periodontal lesion hardly cured by simple medical treatment should be carefully explored through pathological examination, and it was presumed that DLBCL is closely related to the chronic inflammatory periodontal lesions recruiting mucosa-associated lymphoid cells in older patients. It was also suggested that DLBCL be differentially diagnosed from T-cell lymphoma, Burkitt’s lymphoma, and Hodgkin’s disease, etc. with immunohistochemical determination of tumor cell subtypes as soon as possible in order to be treated with appropriate therapy.