Carcinoma ex pleomorphic adenoma is rare, aggressive, poorly understood malignancy that occurs in the salivary glands. The patient was 70-year-old male presenting ovoid elevated submandibular mass that has been present for 30 years and rapid growing for 6 months. The initial cytologic finding by fine needle aspiration biopsy showed highly malignant cells and the radiologic finding revealed a $3.6{ imes}3.7{ imes}4.8cm$ sized mass with cervical lymphadenopathy. After wide excision of submandibular gland and modified radical neck dissection, the histologic examination revealed that tumor was composed partly of a benign pleomorphic adenoma and partly of an adenocarcinoma component with areas of calcification and frequent angiolymphatic invasion. We present a rare case of adenocarcinoma ex pleomorphic adenoma with tumor thrombus in the submandibular gland.