This study examines how the Burmese are integrated in the social system of Mae Sot, a Thai town in the Thailand-Myanmar Borderland, although they lack proper legal status It attempts to overcome the conventional description of them as victims and thus passive actors. It reveals that the Burmese, regardless of whether they are illegal residents, constitute the border society as prominent members. This study suggests that it is quite necessary to take into account illegality or informality as a norm to achieve an adequate explanation of societal formation in the border town. Furthermore, it proposes that the integration of these legal and illegal parts gives a holistic understanding of the society. This study first traces the historical formation of the border social system. It discovers that the multitudinous migration in the late 1980s abruptly expanded the social system and subsequently increased illegality or informality. It goes on to explore the border social system in the sectors of administration, economy and education. This study discloses that the state does not monopolize governance in the town, observing that other regimes such as migrants` own firm of administration and an international refugee regime also take port in the governance In dealing with the town`s economy, this study focuses on Burmese smugglers and border trade. It points out that smuggling is a part of regular economic activities in the border and much volume of goods is traded in informal ways, which constitutes legitimate border trade in its own measure. This study pays special attention to migrant schools because others came to have their own educational institution. It observes that migrant school are positively recognized by the stare as regular educational organizations. Throughout this study, it is shown that informality or illegality is very much contained in the border social system It is crucial to integrate both informality or illegality and formality or legality to have a holistic understanding of the town.