The article presents causes of change in expressing identity of the Tai Dam ethnic group due to power relationships between the governing and the governed and the concept of nation state. From the field investigation and documentary research of the author, it was found that identity expression of the ethnic group had changed due to the power relationships between the governing and the governed in various temporal and social contexts, as the ethnic group by birth, as naturalized Thais in the nation-state context, as well as established Thais required to express their ethnic identity in annual festivals. In the past, these changes were designated by outsiders; however, they are now by choice of the ethnic group. These power relationships have been continuing into the future, which means that the changing identity expression of the ethnic group will continue and no one knows when they will end.