Jeng Xuan's (鄭玄) elucidation on The Three Books of Decorum (三禮) has long been considered a milestone marked in the history of Chinese Classics study. In his time, Jeng came last and benefited by the labors of his predecessors. His prestige, in fact, incarnates all the glory of Classics study in the Hahn Dynasty. In his annotation on The Book of Rites (禮記), Jeng frequently mentions that a certain institution lays foundation of Chou Kong's (周公) reign of peace and order. According to Jeng, Chou Kong's administration even followed certain conventions prevalent in the preceding Three Dynasties, i. e., Yu, Xia Yin (虞，夏，殷). This tripartite article is an attempt to probe into the origin and peculiarities of Jeng's viewpoint. The first part focuses on the evolution of Decorum study in the Eastern Hahn Dynasty. It profiles the general status of certain schools, e.g., the development of Chou Decorum (周禮) study prevailing in the Archaic School. In the second part, I mean to induct some laws based on Jeng's annotations on "Yue Ling" (月令) and "Wang Jir" (王制). These conventions, according to the induction, digest and absorb the essence of Chou and Yin's administration. This part also surveys the difference between my observations and the Modem School's study of Decorum. The last part of this article deals with Jeng's hidden intention to put forth his theory with which this Classics master gives an impetus to his following scholars. To conclude, my argument defies the general notion that Jeng combines the Archaic School's commentaries with the Modern School's and creates his own school. It has been proved that Jeng favors the Archaic School. The bias is clealy seen in his commentaries of the conventions in Chou Decorum.