Following the approach of institutional economics to study the interaction between the changing formal rules and the evolving informal constraints, alongside the dynamics leading to industrial evolution, this paper will firstly examine the particular social and economic settings which bring about the informal constraints for the business practice of Macao's casino junket operators, and the feedback from the system which in turn triggered the evolution of the informal constraints since the middle of the 1980s. Extending from our understanding of the custom and practice of this business in the "old Macao", this paper will further discuss the role of the recently promulgated Junket Operator Regulation in the transition of Macao's casino industry from a culturally defined to a more formally regulated business. In this context, impact from the change in the formal rules on the evolving informal constraints, and the dynamics giving rise to the changing structure and performance of the casino industry will also be explored.