Traditional computer-aided design (CAD) systems are insufficient to support the upstream conceptual design process of mechanical products. A process model for conceptual design based on functional surface decomposition and reconstitution was proposed. The model consisted of three layers: function layer, functional surface layer and form layer. The mapping mechanism among function, functional surface and form was created and the steady mapping relationship from function to form was realized by introducing functional surface as the functional and structural information carrier. It is helpful in developing the computer-aided tools to support conceptual design. A numerical controlled milling machine was applied to demonstrate the proposed process model. 