The theoretical concern of this report is two-fold. One is the flexibility of the ethnic identity, and the other is the possibility of an “alternative nationalism from the bottom”. The focus is on the relationship between “custom” and “Lahu-ness” in the process of religious change in two Lahu villages that are influenced by Christianity and by Chinese religious traditions. It is found that the criteria for Lahu ethnic classification are flexible. Customs and traditions define “Lahu-ness” regardless of religious beliefs. At the same time, “Lahu-ness” is defined not only by customs and traditions but also by birth and by married couples meeting the criteria.