Determining the liability basis of professionals (such as doctors, lawyers and engeeniers), is one of the most important matters in different legal systems. The problem regarding determing the liability of these people is, on one hand, the differences exist between service providng and product manufacturing which makes it difficult to provide a general principle regarding civil liability for all of them. On the other hand, their clients, due to lack of science and skill, could not prove professionals’ fault. The social responsibility of these people, make determing the basis of their liability, more harsh. Although in both French and Common law legal systems, the main theory regarding the professionals’ liability is based on fault, there is some solutions to impose strict liability in some cases in order to protect their clients. Knowing this solutions and ocalizing them, will lead us to the general principle of professionals’ liability under Iranian legal system. In order to sloved the aforementioned problem in Iranian legal system and for the porpuse of balancing between professionals’ liability and social responsibility (mossion), in case service providing ends in goods manufacturing, such as architect and pharmesict, “ Etlaf” and “ Tasbib” shall be considered as the basis of the liability. In case of mere service providing, where there is a breach of an “ obligation by result” , no-fault liability and where there is a breach of an “ obligation by means” , fault base liability seems suitable. Any loss to third parties is included in the latter one.