It is necessary to evaluate the network behavior of users in grid computing in order to resolve the security problems due to the dynamics and uncertainty of the grid environment. Thus, evaluation of the user’s behavior can reflect his reliability. This paper discusses the security requirements in grid environment, offers the correlative definitions of trust, According to the former analysis, a new grid trust model is proposed. This model is used to discriminate the trust relationships between users in the grid environment. It provides a method to analyze the trust degree of users and the report in the grid environment. The two different trust relationships within and between domain are handled with different approaches. This dissertation studies how to build a trust model for grid computing in order to find out malicious users and improve security and reliability of system.