75% of cancer patients have pain, which can greatly affect their quality of life. In cancer patients, not only physical pain but also total pain must be treated. Cancer pain can be classified as nociceptive pain or neuropathic pain according to neural mechanism. It is important to consider the underlying cause and mechanism. In 80% to 90% of patients, pain can be alleviated by the WHO (World Health Organization) guidelines, which have been established as standard treatment for cancer pain. Therefore, all physicians involved in cancer care should become skilled in this analgesic therapy. This article mainly explains the characteristics of cancer pain and the WHO guidelines.