Abstract: Law and custom often co-exists in society, and with mutual transformation and reflection. The studies of this subject is unified in a dialectical and continuous way for the two sides of a same thing. Based on custom, law embodies its efficiency, but it could be enforced only by the law which can be transformed into custom. In people’s daily life, custom is unconsciously to be influential. As another clearer expression of law, custom is a morphology of order coming out by itself with the help of natural formation and fixation to transmit generation after generation. Custom is modified by culture, and dominated by unconsciousness, while law is expressed by words, and sensed consciously. As an object of legal studies, custom should be traced from its transformation and forming into law. Custom and law influence each other, and to be mutual-subjects, while peeping their own unique being of value. They occupy their individual field of culture, form different reason and praxis of communication, and influence the way of formation of our social order.