The thesis investigated the changes in the economic and ecological systems as well as the state of economic development that affected the lifestyles and culture of the Mlabri ethnic group at Bo Hoi Village, Tambon Yap Hua Na, Wiang Sa District of Nan Province and Ban Boon Yuen, Tambon Ban Wiang, Rong Kwang District of Phrae Province. It is found from the investigation that economic impacts included the reduction of forest areas affecting their livelihood, limited fertile land and dwindling natural resources affecting their food sources. Cultural contact between the Mlabri and the Hmong changed the ethnic group’s economic system. It was changed from “silent” trade to paid labor. As regards external culture, the change in dress was most evident. However, the National Social and Economic Development Plans had little impact on the group. The group wanted the state to help it lead an independent, nomadic life. Therefore, state development officers did not have an important influence on the group. Only non-governmental development officers were in close contact with the group for religious purposes, which led to a change in some of their habits.