In this study, we used the broaden-and-build theory and emotional response theory as the framework to examine the effects of instructors' positive emotions on student engagement and critical thinking in U.S. and Chinese classrooms, as well as the mediating role of students' positive emotions in their relationships. MANOVA results revealed no significant cultural differences in student reports of their instructors' and their own positive emotions across the two cultures, but instructors from both cultures displayed more positive emotions than what students felt or experienced in the classroom. Hierarchical regression analyses results also showed that instructors' positive emotions had positive effects on student behavioral and cognitive engagement and critical thinking in both cultures, but the effects were largely mediated by students' positive emotions. Consequently, this study highlights the importance of instructors' and students' positive emotions in the classroom.