Introduction. Use of complementary and alternative medicine (CAM) in cancer patients is increasingly acceptable worldwide, but most of the studies were surveyed from developed countries. In this study, we evaluated the first and large cohort of cancer patients with CAM use in Thailand. Materials and Methods. A self-administered questionnaire was completed by 248 cancer patients attending outpatient radiotherapy unit at Ramathibodi Hospital. Results. The prevalence of CAM use was 60.9%. The most frequently used CAM were dietary/vitamin supplements (56.9%). Independent predictors of CAM use were high income (P < 0.001) and cancer type (P = 0.019). About half of the patients (51%) reported positive effects from CAM use. Nevertheless, 9.4% of the patient also reported side effects. The majority of patients (58.3%) did not disclose their use of CAM to their doctors because they felt that it was not necessary for doctors to know (65.9%). The average spending for CAM use was 200 USD/month (range, 10-1,000). Conclusion. Although the cost for CAM is relatively expensive, the prevalence of CAM use in cancer patients in Thailand is high particularly, in patients with higher income. Therefore, all clinical oncologists should be concerned about the use of CAM during evaluation of the cancer patients.