Knowledge building is an innovative pedagogy aiming to transform students into expert-like knowledge workers. The purpose of this study was to foster teacher-education students' design performance (i.e., observation, synthesis, ideation and prototypes) in a knowledge-building environment. The participants were 38 college students from a Taiwanese university. They were randomly assigned to eight groups. Each group used the Knowledge Forum--a computer-supported collaborative knowledge-building environment--to develop and discuss the design products of their choice. Data analysis focused on the groups' online knowledge-building activities, online design thinking processes, and the relationship between them. In general, knowledge-building activities facilitated students' design thinking. Specifically, we considered the extent to which student groups' sustained online engagement and increased knowledge had a major impact on their design performance. Ways of applying knowledge-building principles to foster effective design processes are discussed.