This is an attempt at examining the historical relation between tribe and caste in Bengal. Usually, caste system is considered as the most important and fundamental social frame, at least, up to the advent of Muslim in India. However, the present author has had since long a serious doubt about this kind of understanding. The author’s field of research locates in the frontiers of the Bengal plains and the majority of the population there belongs to a caste called the Rajbamsi. The Rajbamsi is a relatively new caste formed after the 16th century A.D., and the people there have a very vivid memory of their previous tribal name, the Koch. If we look at the social history of Bengal, we can easily pick up many cases of similar transformation of a tribe into a caste or castes. The full-fledged caste system was established in Bengal only in the thirteenth century. Therefore it is grotesque to think of Bengal society in terms of caste hierarchy for the period previous to that date. Then, what was the social foundation of Bengal before the thirteenth century? The natural answer to this will be the tribal systems. However, this identification remains only guesswork, unless we can provide sufficient evidence and reasoning to support this proposition. This paper exactly aims at this difficult task. The task is difficult, not only because of paucity of the historical evidence to prove it, but also the idea contradicts the conventional ‘common sense.’