This paper discusses the design modification issue in a collaborative assembly (co-assembly) design environment, which enables multiple geographically dispersed designers to design and assemble parts collaboratively and synchronously through the Internet. An assembly representation model, viz. feature-based hierarchical co-assembly representation, is proposed to resolve the co-assembly design issues. In order to realize the design modification, a design modification propagation control mechanism is proposed. A system framework that is suitable for realizing the design modification is also proposed and developed. Finally, the detailed design modification propagation control mechanism is demonstrated through a case study.