This paper reports a case of infantile fibromatosis occurring at the alveolar portion of the mandible. Imaging findings were analyzed in detail.<br>The patient was a 7-year-old girl, complaining of a painless swelling of the gingiva at the molar region of the left mandible, which had been noticed two days before the first consultation. There was an elastic hard mass at the first molar to retromolar region. CT scans revealed a soft tissue mass protruding from the alveolar crest and invading the mandible. The mass showed more intense enhancement than the muscle and the peripheral portion of the mass was more enhancement than the central portion of the mass, including a tooth in it after contrast infusion.<br>The alveolar bone was destroyed from the crest to the region below the apex of the first molar. The margin of the lesion was irregular. There were lacerations of the lingual cortex that bulged slightly. A malignant tumor with low malignancy was diagnosed based on imaging findings. Histological diagnosis was infantile fibromatosis. The lesion was supposed to have originated from the soft tissue at or around the alveolar crest.