This chapter seeks to provide an analysis of how norms of public international law were received into Thai legal system through the exploration of the Thai history beginning from early Rattanakosin era to present. Thailand, (formerly known as Siam), has long been involved in making international law since early Rattanakosin period. The first well-known treaty was a treaty of comity and commerce concluded in 1826 between Kingdom of Siam and the United Kingdom of Great Britain and Ireland (Burney Treaty). Twenty nine years later, the 1855 Treaty of Friendship and Commerce between Siam and Great Britain (Bowring Treaty) was signed. With great impact that those two treaties had in the context of the Siamese, this chapter will analysis ways in which these two treaties have exerted its influenced on the its domestic legal culture. Given the fact that reception of international law into domestic legal system is largely a matter of constitutional law, Thai constitution remains silent about the issue of how customary international law can be incorporated as part of Thai law. However, the Thai domestic courts have ever dealt with this situation when they were asked to render the judgements concerning the doctrine of hot pursuit and the immunity from jurisdiction. This chapter will describe the approach that the courts employed in order to draw a conclusion of how customary international law is received into the Thai legal system.