Dermoid cyst is a benign neoplasm, so called by teratoma that is derived from both ectoderm and mesoderm during embryonic period. The majority of dermoid cysts are found in the ovaries and approximately only 7% of dermoid cysts are found in the head and neck. The growth of dermoid cysts in the floor of the mouth is considered a rare condition. We report one case of huge dermoid cyst in the floor of mouth suffered with double chin appearance and difficulty in pronunciation and respiration. The cyst was excised successfully only intra-oral incision. Clinical progress was uneventful and postoperative recovery excellent without recurrence.