Abstract: With the swift and violent development of hardware, software has become more and more complex. How to rapidly develop maintainable, extensible and adaptable software that meets the changeable requirements is becoming a pressing problem. As everyone knows, component\|based development offers a better means to solve the problem. The critical point of that way is the design of reusable component. Much effort has been made to design component and to optimize component structure, without sound results. To obtain well structured components and component library, a method for reusable component design is presented in this paper, which is better than the existing method. At the beginning, several existing design methods of reusable component are discussed and evaluated briefly. A component model is defined and the common designing steps are given, and then the new method including some algorithms is introduced in detail, which combines quantitative analysis with qualitative analysis. With that method, one can identify and optimize the inheritance and aggregation structures of class components quickly. And the automatically supporting tools can be constructed easily. After that, some problems existing in the construction and evolution of component library are discussed. Several cases in practice are also presented briefly.