A 40-year-old man presented with right eye axial proptosis and ophthalmoplegia for 3 months. Imaging study showed a right intraconal mass with the erosion of the orbital floor. Incisional biopsy revealed mucoepidermoid carcinoma. Nasal endoscopy was normal and systemic tumor screening was negative for a primary source. The patient underwent right orbital exenteration, uncinectomy, nasal and maxillary mucosal biopsy. Malignant cells were found present in the mucosa of maxillary sinus roof and uncinate bone. The postoperative positron emission tomography scan showed residual active lesion in right orbital apex and maxilla but no primary lesion elsewhere. The patient subsequently underwent 35 cycles of postoperative radiotherapy. Primary mucoepidermoid carcinoma of the orbit is rare and typically arises from the lacrimal gland or sac. Those tumors not arising from lacrimal apparatus should be presumed metastatic in origin, and the thorough systemic survey should be undertaken in the search for the primary tumor.