The present study attempted to investigate whether young learners who were new to knowledge building approaches could work towards advancing both individual and collective knowledge, and whether knowledge building could be beneficial to both high-achieving and low-achieving students. Findings reported in this paper are from one and a half-year design research for science learning in one primary school in Singapore. In this study, we closely examined the design and enactment of the Knowledge Building Community model in one class with high-achieving students and two classes with mixed-ability students. The research consists of two phases: Phase I Cultivating a collaborative knowledge building culture and Phase II Progressive Knowledge Building using Knowledge Forum. Data were collected from multiple sources, including knowledge assessment, conceptual understanding tasks, and the content analysis of Knowledge Forum postings. The results in Phase I show that while it is critical for students to monitor and build knowledge for their own understanding, they had difficulties developing such skills. In both phases, we found positive impacts on academic achievements showing improvement of student understanding in the course of reflective thinking and progressive inquiry. Overall, quantitative data suggest that the collaborative knowledge building environment was beneficial for both high-achieving and low-achieving students. We conclude by discussing some of challenges and issues in designing collaborative knowledge building environments for young learners with diverse abilities. (Contains 2 figures and 6 tables.)