Northeast India surrounded by Bangladesh, Bhutan, China and Myanmar is the place of multi-ethnic society. In the plain area of the Brahmaputra valley in Assam, mixing of different ethnic groups founded the basis of present society and culture, but also often caused the conflict and social disorder. This study aims to discuss how these people have traditionally coexisted in the Brahmaputra valley from the viewpoint of ecological environment. The study area is Lakhimpur district in eastern Assam where many ethnic groups like Ahom, Mishing, Kachari, Khamti, and Bengali live in. Both primary and secondary data were collected to investigate the village location, village structure and livelihood pattern by ethnic groups. From the GIS analysis, it was found that people in the study area lived in different ecological zone by ethnic groups. Each group has the unique livelihood pattern based on local ecosystem. For instance, the Ahom living in plain zone and the Mishing and Chutiya living in river island zone grow different crops in the different season of a year. They can interact with each other by exchanging their livestock and labor services. The fluctuation of micro environment of the Brahmaputra floodplain may enable the coexistence of the different ethnic groups.