It is well established that hyperbaric oxygen can enhance the sensitivity of cancer cells to radiotherapy and chemotherapy. It is theorized that cancer tissues are less sensitive to chemotherapy and radiotherapy when they are hypoxic, and their malignant behavior increases. Hyperbaric oxygen increases oxygen and free radicals in cancer tissues, and chemotherapy and radiotherapy kill cancer cells dependent on oxygen free radicals. In addition the mechanism of hyperbaric oxygen sensitization to chemotherapy and radiotherapy is also related to the alteration of cell cycle by hyperbaric oxygen. Therefore, hyperbaric oxygen is designated as an adjuvant to radiotherapy and chemotherapy applied to malignant tumors. There is still disagreement about whether hyperbaric oxygen promotes or inhibits tumor growth. For the sake of prudence, no one has proposed to remove hyperbaric oxygen as a relative contraindication for malignant tumors so far.