High assurance systems have evolving requirements and, hence, the definition for assurance requirements should allow evolution. We recommend the use of ontology to capture such evolving requirements. In this paper, we introduce the concept of trustworthiness to include dependability and some additional high assurance attributes. We develop an ontology of trustworthiness aspects and an ontology of system entities. Based on these ontologies, a framework for trustworthiness assessment is constructed. This framework supports systematic and well guided trustworthy assessment from the individual system entities to the overall integrated system. It also offers the potential for automated trustworthiness assessment.