Research Findings: This study compared the peer interaction patterns in mixed-age and same-age Chinese preschool classrooms using videotaped behavioral observations. A total of 179 children (ages 3-6) from three mixed-age classes and three same-age classes were involved in this study. A total of 170 valid videos were obtained, including 85 videos from the mixed-age (M[subscript age] = 58.99 months, SD = 10.09, 34% girls) classrooms and 85 from the same-age (M[subscript age] = 57.74 months, SD = 10.84, 43% girls) classrooms. The results revealed that: (1) there was a significant age effect in the frequency and duration of peer interactions, with older children having more and longer interactions; (2) there was a significant between-group difference in the duration of peer interactions, with MA having a longer duration than SA; (3) there was a significant same-gender preference with children preferring interacting with the same gender peers; (4) the MA classrooms had significantly more positive interactions and significantly less neutral and negative interactions than the SA classrooms; and (5) the MA classrooms engaged in significantly more dominating and less neutral interactions than the SA ones. Practice or Policy: These findings have implications for early childhood teaching, classroom organization, and classroom management.