Public-Key Infrastructures (PKIs) are essential for providing electronic payment security services such as authentication and privacy in open networks like Internet. The purpose of this paper is to propose a Description Logic-based approach to modeling and reasoning about PKI trust domain. In the formalism, concept model is used to formalize PKI entities and trust domain, which gives most precise definitions of different PKI entities and is more expressive than that of first-order logic-based approach. This approach can model hierarchical and mesh PKI structure, as well as multi-domain PKI connected with trust mechanisms such as cross-certification or bridge CA. A noticeable feature is that it can also be extended easily to integrate with PMI to form a unified framework for authentication and authorization.