CT findings of proven 25 malignant tumors of the maxillary sinus were retrospectively analyzed to be of help in the diagnosis and treatment. The results were follows: 1. Average age was 54 years old, and eighteen were males and seven were females with a ratio of 2.6:1 2. The most common histopathologic feature was squamous cell carcinoma (19 cases) and others were two cases of adenoid cystic carcinoma, one case of malignant fibrous histiocytoma, mucoepidermoid tumor, histiocytic lymphoma, unidentified malignant tumor. 3. CT findings were sinus opacificaqtion (4%), soft tissue mass (92%), low densities within soft tissue mass (44.%), air densities within soft tissue mass (24%), osteosclerosis (4%), bone destruction (92%), bone displacement (32%), fat plane obliteration (76%). 4. CT in the malignant maxillary sinus tumors approved the value in evaluation of tumor extension to nasal cavity, ethmoid sinus, orbit, infratemporal fossa, pterygopalatine fossa, pterygoid fossa, pterygoid muscle, cheek skin and intracranial cavity. 5. Twenty four cases (96%) were stage Ⅲ, stage Ⅳ according to AJCC TNM classification. 6. Bone findings were destruction, displacement, sclerosis and most frequent site of bone destruction was the medial wall of the antrum(92%). 7. Tumor growth pattern showed destructive pattern in 18 cases(72%), and squamous cell carcinoma showed destructive pattern. (P<0.05)