Pen-Based user interface is a primary Post-WIMP (window icon menu pointer) interface, and it provides natural interactions for users. However, the current toolkits for constructing pen-based user interface are built for single-user tasks, which can not support collaborative work well. This paper analyzes the features of pen-based interaction and collaboration environment. Furthermore, the design and implementation of a toolkit, CoPen Toolkit, which can support the development of collaborative pen-based user interfaces are discussed. CoPen Toolkit offers flexible architecture and extended components, which can support ink-based data, event processing, networked collaboration, etc. Based on CoPen Toolkit, several typical prototypes have been built, and the results show that it can support construction and fast-prototyping of collaborative pen-based user interfaces.