How to embody the requirements of collaboration in designing processes is an important issue in developing collaborative design systems. In this paper, a hierarchical operation model for collaborative design is presented. A mental component called joint endeavor and a set of communication primitives are devised for the half-automated agents in the model to meet some of the requirements in collaborative design. An implementation structure for the hierarchical operation model is also given. A collaborative object diagram design system (CODDS) is introduced as an example of this collaborative design model.