Radiation dermatitis is defined as skin changes after radiation exposure, which mostly follow certain predictable course dictated by radiation dose, timing, and affected individual cells. Acute radiation dermatitis is mostly characterized as mild erythema and edema which occurs hours after exposure, whereas chronic radiodermatitis occurs after months to years, leading to skin fibrosis and atrophy. Conventional radiation therapy utilizes ionizing radiation, especially known as X-rays and Gamma rays. However, recent studies regarding the other categories of ionizing radiation is actively ongoing. Among them, carbon ion radiation therapy using particle beams is cutting edge issue. A 43-year-old man presented with mild pricky erythematous swelling on left forehead, first appearing 3 days ago. Diagnosed with palate cancer extending to skull base a year before, he was treated with carbon ion radiation therapy (16 times, 64Gy) 9 months ago at Japan. We took a skin biopsy, which turned out to be mild spongiosis with perivascular, periadnexal lymphocytic infiltration. Under the clinical diagnosis of radiodermatitis, patient got no further treatment. Unlike common course of acute radiodermatitis which appears 2-24 hours following exposure, the patient experienced relatively long latency period of 9 months. Proper patient notification regarding adverse dermatological outcome is therefore necessary for whom receiving carbon ion therapy.