In enterprise architecture, the goal is to integrate business resources and IT resources in order to improve an enterprises competitiveness. In an enterprise architecture project, the development team constructs a hierarchical model that represents the enterprise. Our goal is to develop a hierarchical object-oriented model that can serve as enterprise model. The state of the art in hierarchical object-oriented models is such that no models are suited for that purpose. Existing hierarchical models either represent the hierarchy in an inconsistent way, or focus on only one system of interest, or inadequately model actions, or lack a complete meta-model to keep the traceability across the hierarchy. In this paper, we present a meta-model that defines a hierarchical object-oriented model of systems such as those found in enterprise architecture. Millers organizational levels and the Reference Model of Open Distributed Processing are the foundations of our meta-model that is currently implemented in a web-based CAD tool using Java technology. This paper ends by presenting an example developed with our CAD tool.