Conceptual process planning is an activity for designers to evaluate manufacturability and the manufacturing cost in the early design stage for mechanical parts production. Since major manufacturing costs of a product are committed in product specification and design, it is critical to be able to assess manufacturability and cost as early as possible in the design process. At the National Institute of Standards and Technology, the Design and Process Planning Integration (DPPI) project addresses the need for improving communication between conceptual design and conceptual process planning activities. Documenting the DPPI foundation, this paper provides a definition of conceptual process planning and describes its functions in an activity model. Also, this paper describes the conceptual process planning prototype system that has been implemented and integrated with a conceptual design system. The prototype system validates the definition, the activity model, and the integration between process planning and design in the early product development stage.