Pen-Paper metaphor is a universal and fundamental way for capturing daily experiences, communicating ideas, recording important events, conducting deep thinking and visual descriptions. This talk focuses on our research in pen-based user interface, especially Interaction Characteristics of Pen UI, Pen UI Paradigm and Pen-based Interaction Techniques, and Pen UI Software Architecture and Applications. The ink is regarded as a type of first-class data type. We have researched stroke parsing and recognition algorithms, intrinsic structure understanding of ink, and semantics model of ink. Moreover, we present a new interaction paradigm named PIBG, in which user action is changed from mouse pointing to pen gesture. Based on the PIBG paradigm, we have built the software architecture and a toolkit to support the development of pen-based interaction systems. Furthermore, we try to apply a scenario-based design approach in the whole software development process. At the last, Several application demos, such as PenOffice, SketchPoint, MusicEditor, CreativePen, will be presented.