The contract of treatment is of great importance due to its relationship with two fields of medicine and law. Considering the fact that essentially the legal relationship between the health professionals and their patients is provided by the contract concluded between them and sometimes due to the ambiguity of laws and etc. the scope of their liability is disputed, firstly, determining the legal nature of treatment and secondly taking advantage of common and specific rules relating to the applied contract will be of great importance in resolving the possible future disputes. Thus in this article, after studying the definiteness or indefiniteness of the contract of treatment, we have set about studying the aspects of the theory of definite contract which includes the theories of contracting, advocacy, renting individuals, the contract of work and reward agreement nature of the contact of treatment and eventually the selected theory concerning this issue has been presented and the most important characteristics of this contract have been discussed.