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The diagnosis of ameloblastic carcinoma is based on a combination of cytological features of malignancy and histological pattern of an ameloblastoma. We report a very rare case of ameloblastic carcinoma arising from a pre-existing ameloblastoma of the maxilla with challenging diagnostic aspects due to atypical histology. A 74-year-old female with a swelling in the right maxillary region was referred to our hospital in March 2015. Intraoral examination revealed a 45 × 30 mm sized swelling extending from the right upper first premolar to the maxillary tuberosity. Radiographic examination revealed a tumor mass with heterogeneous enhancement from the right maxillary alveolar area to the maxillary sinus, invading the pterygopalatine fossa under the cranial base, with bone destruction of the alveolar bone, floor of the maxillary sinus, and pterygoid plate. Incisional biopsy results revealed round or short spindle cells with clear cytoplasm, with low mitotic activity, cellular atypism, and Ki-67 labeling index. A low grade malignant epithelial tumor was suspected. Tumor resection with hemimaxillectomy and partial mandibulectomy was performed in April 2015. The surgical specimen was composed of diffusely distributed spindle cells and clusters of round cells with clear cytoplasm, similar to the biopsy findings. Although cellular atypism was scant, Ki-67 labeling index was high in most sections. Ameloblastoma was detected in part of the tumor and a definitive diagnosis of ameloblastic carcinoma was made. The patient died of metastatic colon cancer 3 years and 10 months post-operatively; there was no evidence of recurrence and metastasis of ameloblastic carcinoma in that duration.