Abstract For assuring dependability among systems, it is necessary to describe not only system architecture, but also assurance argument between elements of systems. The composite dependability means for assuring dependability between components of a system. Several approaches have been focused to show the assurance of one single system. Moreover, a unified method to describe both assurance case and system architecture in the same diagram has not been known. This paper proposes a unified method to describe assurance case and system architecture by using an Enterprise Architecture modeling language, Examples of the proposed method are also described. The result shows the method is effectively applicable for assuring dependability of business, application, and technology architecture.