The factors contributing to the cultural assimilation of the So ethnic group with northeastern culture include gender, age, marital status, education, economic status, participation in social activities, and contact with outsiders. It is found that half of the So can speak the northeastern dialect well. They prefer to speak the dialect or standard Thai to their ethnic language outside of their communities. Four in five consider that it should be voluntary to move into either spouse’s home after marriage. One in three does not believe in or practice animism and folk healing. Half of them like to be ordained as monks and regularly offer food to monks. Regarding cultural assimilation with northeastern culture, it is found that two in ten are assimilated at a high level, seven in ten at a medium level, and one in ten at a low level. Women and those with a better economic status, have more frequent participation in social activities and more frequent contact with outsiders and tend to be more culturally assimilated. However, age and education are not significantly related to their cultural assimilation.