Based on the following five aspects such as, the traditional factors, trendof though, fimction of law, system of law and law itself, this paper discussesthe idea of law in Han Dynasty. In order to fit the need of the operation ofpolitical system, also to meet the need of safeguarding the traditionaI socialorder, Li and Law were further united by the promotion of Confucian scholarsin Han Dynasty. Li influences the existence of law not only in spiritual essencebut also in detailed constitution. Accordingly, law plays an important role inconsolidaing the society on the higher level with the combination ofpunishment and instruction and the unification of legislation and comparativeflexibilty ofjudicature. However, the law enlnyes its value rationality at thecost of sacrificing its formality rationality, which is the inevitable result of theevolution of the idea of law in Han Dynasty.