Knowledge diversity describes group members' differences in terms of prior knowledge in a domain. The purpose of the study was to investigate whether knowledge diversity would impact students' engagement in small-group learning in a science classroom. A total of 45 seventh-grade students were recruited to participate in the study in which two experimental conditions were compared: low-prior-knowledge groups (all low-prior-knowledge students) versus mixed knowledge groups (low-prior-knowledge students with one knowledgeable student). Participates were randomly assigned into six low-prior-knowledge groups (24 individuals in total) and five mixed knowledge groups (21 individuals in total). Engagement, as well as group performance, was measured. The results of a series of independent-samples "t" test demonstrated that the mixed knowledge groups had significantly higher behavioral, emotional, and social engagement and better group performance than did the low-prior-knowledge groups. This implies that even having one knowledgeable student could enhance students' engagement in a science classroom.