Although the potential of peer assessment activities in promoting student learning and fostering student cognitive development has been widely studied and well documented across the world, it is unclear how peer assessment may benefit students of diverse achievement levels. This study examined this issue via a mixed methodology approach that combined students' project scores and survey responses with a qualitative interview technique. Findings of this study suggested that peer assessment activities employed in this study had a differentiated impact on students' learning and perceptions. Students in early learning development stages showed more learning gains than high achieving students in terms of point increase in their grades. Nevertheless, students across diverse achievement levels generally held positive attitudes towards their peer assessment experience. [Note: The issue month (1) shown on this PDF is incorrect. The correct citation is Jul 2011.]