Reading motivation can greatly impact reading comprehension, but it tends to diminish in and beyond elementary school. This study employs knowledge building pedagogy to advance reading motivation and comprehension in an elementary Chinese language arts class. Participants were twenty-four third graders who spent one class period (ie, 40 minutes) for 18 weeks in Knowledge Forum to study 10 lessons in a reading textbook. Data mainly came from: a reading motivation questionnaire, online interaction logs, online discourse and PILRS reading comprehension assessments. It was found that knowledge building enhanced children's reading motivation (particularly in terms of reading competence) and reading comprehension (especially at the higher levels). Most students demonstrated spontaneous engagement in advanced online activities demanding high-level agency. A correlation was also identified between children's motivation to read and more advanced online discourse behaviours requiring higher agency, leading to a deeper understanding of reading. Implications for fostering a motivated reading community that values collective knowledge advancement are discussed.