Mucoepidermoid carcinoma is a malignant glandular epithelial neoplasm characterized by clear mucus producing cells and epidermoid cells including cyst formation in tumors. It usually originates from salivary gland, however, it seldom occurs centrally within bone. Out of all the mucoepidermoid carcinoma cases reported in literature, the origin of intraosseous mucoepidermoid carcinoma is still unknown. In July 2006, a 64-year-old woman was referred to our hospital with chief complaint of non-painful swelling of the anterior region of the mandible. Previously, in July 1998, she had been referred to our hospital with the same complaint of the mandible. Clinical and radiographical findings showed the cystic lesion of the mandible, therefore, an incisional biopsy was performed. Consequently, the histopathological diagnosis was glandular odontogenic cyst. Although a cystectomy under general anesthesia was recommended, she opted not to follow through any treatments due to her personal circumstances. The alveolar gingiva was almost normal, but the bony expansion of labial and lingual sides of the mandibular anterior region was noted. The size of the expansion was approximately 38x26mm in size. CT scan and MRI findings suggested two-locular cystic lesions in the mandible and soft tissue tumor in floor of the mouth. The biopsy of the mandible lesion was performed on 19 July 2006, and the histopathological finding was intraosseous mucoepidermoid carcinoma. In September 2006, segmental resection of the mandible accompanied with resecting soft tissue tumor in floor of the mouth and reconstruction using free fibular flap were done. The pathological diagnosis of resected specimens was low to intermediate grade mucoepidermoid carcinoma. The lesion also demonstrated expansile growth within bony mandible and local absorption of lingual cortical bone. Furthermore, the sublingual gland tissues partially turned into tumor which became continuous with the intraosseous lesion of the mandible. Three months after the operation, there were no symptoms and findings suggesting local recurrence. From the additional histopathological analysis including immunohistochemical stainings, we could not prove to differentiate the origin of the tumor which derived from the mandibular cyst or the sublingual gland. However, clinical and radiographical findings in long-term observation strongly suggested the possibility of transformation of glandular odontogenic cyst into mucoepidermoid carcinoma.