Abstract. Trust is hard to establish in a service-oriented grid architecture because of the need to support end user single sign-on and dynamic transient service. In order to enhance the security by the Grid Security Infrastructure (GSI), this paper proposes a two-level trust model and the corresponding trust metrics evaluation algorithms. The upper level defines the trust relationships among Virtual Organizations (VO) in a distributed manner. The lower level justifies the trust values within a grid domain. This novel model provides an integrated trust evaluation mechanism to support secure and transparent services across security domains. It is flexible, scalable and interoperable. We design the implementation of embedding the trust scheme into GSI. At this stage, we achieve additional authentication means between grid users and grid services.