Grid computing systems are a distributed geographical environment where the resources of entities are shared between each other and defining the type of entities is fully in control of their owners. Each entity can apply kind of limitations to the use of its resources by other entities. Because the resources of the non homogeneous grid belong to the various places with various business policies, it is very important and complex to manage the behavior trust of the resources in return to the requested works. In this paper a new method is proposed to the ability of probability theory for managing the behavior trust in grid computing systems.