Quasi-contract rooted in the roman law,and was inherited by the civil law and the common law,but there are many differents in the quasi-contract between the civil law and the case law.

By virtue of the theory of quasi-contract in Roman law,we can not only explain the obligation conveyance under the situation of issuing bill of lading,but also solve the problem on how to determine the litigation right in legal practice.

The imperfectness of contracts causes the inevitable existence of accounting policy choices,and the different contract costs lead to different motives in accounting policy choice.

According to the minimization of contract costs and supervision contracts,two factors influencing voluntary accounting changes are explained.

The thesis also introduces Positive Accounting Theory based on the concept of contract costs and its three hypotheses ( Bonus Plan, Debt Covenant, Political Cost) of economic motive in the choice of accounting policy.