With the increasing deterioration of the natural environment and the growing trend of regional integration,as single local government has been unable to complete the overall governance of regional environment,the importance of local government cooperation stands out.In cooperation process,as an important part of cooperation cost,transaction cost has an important influence on cooperation process.This paper,selecting transaction cost theory in new institutional economics as the study perspective,analyses local government cooperation process,constructs a theory model and provides a detailed analysis of transaction costs produced in cooperation process and their mechanism of action.The study shows that transaction cost restricts each other and effects the whole process of government cooperation;meanwhile,in the pursuit of minimizing the sum of transaction costs,local government should consider the commonality of regional environmental governance,so as to determine the optimal solution of transaction costs.