Purpose: This study was conducted to provide basic data for developing education and health promotion programs for the prevention of cancer by identifying the relation between optimistic bias about cancer and cancer preventive behavior in Korean, Chinese, American, and Japanese residents in Korea. Methods: Using a questionnaire administered by the researcher, data were collected from a convenience sample of 600, 19 to 64-yr-old male and female Korean, Chinese, American, and Japanese residents in Korea. Data was collected between February 6 and 28, 2009. Results: Scores for optimistic bias about cancer by nationality were: Koreans, -1.03; Chinese, -0.43; Americans, -0.23; and Japanese, 0.05. The cancer preventive behavior scores were: Koreans, 43.17; Chinese, 71.84; Americans, 71.71; and Japanese, 73.97. Optimistic bias about cancer and cancer preventive behavior showed a significantly positive correlation in all participants: Koreans (r=.223, p=.006); Chinese (r=.178, p=.029); Americans (r=.225, p=.006); and Japanese (r=.402, p<.001). Conclusion: The greater the optimistic bias about cancer is, the lower the cancer preventive behavior. The findings suggest that nursing interventions are needed to reduce optimistic bias about cancer and to form a positive attitude towards cancer prevention because an optimistic bias about cancer adversely affects cancer preventive behavior.