The Information Service is a core component in the Grid software infrastructure. It provides diverse information to users or other service components in Grid environments. In this paper, we propose an Information Service architecture model for information management in a Grid Virtual Organization (VO). This Information Service is a hierarchical structure which consists of the VO layer, site layer and resource layer: at the resource layer, information agents and pluggable information sensors are deployed on each resource monitored. This information agent and sensor approach provides a flexible framework that enables specific information to be captured; at the site layer, a site information service component with caching capability aggregates and maintains up-to-date information of all the resources monitored within an administrative domain; at the VO layer, a peer-to-peer approach is used to build a virtual network of site information services for information discovery and query in a large scale Grid VO. This decentralized approach makes information management scalable and robust. Our Information Service has been implemented based on the Globus Toolkit 4 as a Web service compliant to the Web Services Resource Framework (WSRF) specifications. The experimental results show that the Information Service presents satisfactory scalability in handling information for large scale Grids.