The creation and management of ontology data on web sites (e.g. instance data that is used to annotate web pages) is important technical support for the growth of the semantic web. This study identifies some key issues for web ontology data management and describes an ontology data management system, called robinet, to perform the management. This paper presents the structure of the system and introduces a Web ontology data management model that enables a flexible access control mechanism. This model adds rules into the robinet system to utilize the semantics of ontology for controlling the access to ontology data. The implementation of the rule-based access control mechanism and related testing are also discussed.