It is widely acknowledged that decisions made in the early design stages have a greater influence on the final product than those made in the later stages. In the early stages of a conventional design process, composite structures are designed without sufficient consideration being given to limitations of composite manufacturing process. Quite often some of composite designs cannot be produced with special performance requirement or cannot be produced at a reasonable cost. To resolve this drawback and achieve the manufacturable designs for composite product, a knowledge-based decision-making tool was introduced and applied to support the composite design optimization. Firstly, the current development of DFM techniques in composite domain, as well as the main problems in performing DFM for composites is analyzed. Secondly, some of key issues in the composite manufacturing process that should be considered during the composite design process are presented. And then, a knowledge-based DFM system for composite manufacturing process is proposed, focusing on the system decision task, the system framework and the flowchart. Finally, the system is illustrated with a case in autoclave process.