The nature of public projects is a long-term contract made by a group of stakeholders who want to share the project resources on the basis of public demands. There is a complementary relationship between the project contract and the market contract. The project contract is formed by the means of element market. But compared with the market contract, the project contract is depend on the benefits and values produced by the project team's collective action, not on those stakeholders' comparative advantages and societies' working division. If we can explain the nature of public projects and the boundary between project and market, the project can be analyzed, as a result, the project governance theory can be established. Finally, the paper point out the approaches to ameliorate agent construction on the basis of the theory of public project governance.