The purpose of this descriptive study was to explain the levels of knowledge about and involvement in national health policy development by nurses in Thailand. The study used quantitative and qualitative means to gather data about the topic from two groups of professional nurses: 2121 nurses who worked in hospitals around the country and 26 nurse leaders who were members of steering committees in nursing professional organizations. A self-administered questionnaire and an interview guide regarding knowledge and involvement in national health policy were used for collecting the data. The content validity and reliability of the questionnaire were assured. The results showed that almost two-thirds of the sample had a high level of knowledge about national health policy development but that almost three-quarters of the sample had no involvement in national health policy development. The interviews of the nurse leaders showed that some of them had been involved directly in formulating health policy but most of them thought that they had not been involved directly. The results demonstrated that it is essential that nurses understand and be actively involved in national health policy development.