Mucositis and dysphagia are common complications of chemoradiotherapy (CRT) for head and neck cancer. The importance of oral care, which is one form of supportive care for head and neck cancer patients, has been pointed out. In this study, we examined the effect of oral care on head and neck cancer patients treated with CRT. We investigated 34 patients, all of whom agreed to the purpose of this study, with head and neck cancer treated with CRT between August 2012 and March 2013. The patients included 29 males and 5 females. The average age of all patients was 67.4 years. We counted the total amount of intraoral bacteria over time, and found that oral care not only inhibited intraoral bacterial growth but also decreased the amount of bacteria. Patients with a lot of intraoral bacteria were unable to maintain intraoral moisture by oral self-care, and could not manage the pain of oral mucositis. The management of the amount of intraoral bacteria by oral care tended to reduce the symptoms of oral mucositis.