This paper describes a Ubiquitous Performance Support System for Teachers (UPSST) and its implementation model. Personal Digital Assistants (PDAs) were used as the platform to support high-school teachers. Based on concepts of Electronic Performance Support Systems and design-based research, the authors conducted an iterative process of analysis, design, development, and evaluation. Besides the data collected from the two initial users during the iterative process, 12 teachers were recruited for evaluating the functions and interface design of the UPSST. The participants mostly reported positive attitudes toward the functions and interface, but some expressed concerns. Explanations of the concerns and suggestions for future studies are provided. (Contains 5 figures and 1 table.)