From human-computer interaction to human-human interaction and from information sharing to instant collaboration are major advances from traditional standalone and distributed CAD to collaborative CAD. This paper explores model, method and tool to achieve this goal. A human-centered collaborative design model to support human-human interaction is presented. A method to transparently add aware capabilities to single-user CAD applications is outlined. Based on the method, an instant collaboration tool is implemented to support human-human interaction in collaborative design process. The collaboration tool involved some core collaborative issues such as agent structure, communication, group awareness, consistency maintenance and collaborative tasks. We also test the method in a 3D collaborative tool for heterogeneous CAD applications. (C) 2006 Elsevier B.V. All rights reserved.