Abstract: Circadian clock governs 24-hour rhythms in body temperature, blood pressure, circulating hormones, metabolism, and oth-er physiological parameters in humans and most mammals. The circadian clock is regulated by circadian clock genes. For instance, peri-od (PER) is a core circadian clock gene. Human PER includes PER1, PER2, and PER 3, which play a role in cancer suppression. Abnormal PER expression not only causes cancer but also changes cellular sensitivity to radiation and chemotherapeutic drugs. Thus, this theory has provided a new direction for cancer radiotherapy and chemotherapy. This paper presents the research progress on PER in tumors.