Diagrams are used for representing design concepts and constructing function structure in the stage of conceptual design. This paper considers sketches as the language of communication in cooperative conceptual design and presents a tool that enables distributed participants to cooperatively construct diagrams in the form of freehand sketches. The tool is designed and implemented as a multi-agent system, which contains a set of agents to fulfill the requirements, including sketch-based interaction, sketch recognition, communication and coordination. The operating strategies of these agents are also addressed, including communication between agents, sketch recognition algorithm, and semantic consistency maintenance. The proposed approach is implemented in cooperative flowchart drawing.