This study used online activities in a knowledge-building environment to foster design thinking skill. Participants were 38 college students randomly assigned to eight groups in an “Introduction to Living Technology” course at a Taiwanese university. Each group used the Knowledge Forum-a computersupported collaborative knowledge-building environment-to design a product of their choice. Data analysis focused on online knowledge-building activities and their effects on students’ design thinking. In general, knowledge-building activities facilitated students’ capacity for design thinking as a group. Specifically, the extent to which groups sustained online engagement and increased their group’s knowledge had a major impact on their design performance. Ways of applying knowledge-building principles to foster effective design processes are discussed.