Why did the Kung-yang commentary of the Spring and Autumn Annals become the most dominant Scripture in the Emperor Wu reign of the West Han Dynasty? The general theory is that the scripture proposed a system of political ethics that perfectly met the ideological needs of the empire. This paper examines a particular concept in the scripture, jang (讓), to substantialize the above theory. The concept of jang advocated by the scripture means to abdicate the throne to the eldest son of the principal wife (嫡長子) of the emperor, which is quite different from the traditional Confucian ideal of shang-jang (禪讓), that is, to abdicate the throne to the most worthy man (讓賢). The reason for the commentary to propose the new concept of jang can be found by examining closely the political system and expecially the mentality of the local kings of the formative period of the Western Han Dynasty. The significance of the concept jang can thus be revealed.