Pen-Based user interfaces provide more natural interactions for users. However, it is difficult to be constructed because a usable pen-based user interface generally involves multiple areas and interdisciplinary knowledge. In this paper, the design and implementation of a toolkit, named Penbuilder, is described systematically, supporting the development of pen-based user interfaces. It is designed based on the attributes of pen interaction and ubiquitous computing. It offers high-level supports to developers. Based on Penbuilder, several typical prototypes of pen interaction have been constructed, which demonstrates the capability of Penbuilder for easy construction and fast-prototyping of pen-based user interfaces.