Background: Many studies find that physical education (PE) classes in elementary schools rarely meet the recommendation to engage students in moderate and vigorous physical activity (MVPA) at least 50% of the class time. PE teachers are responsible for engaging students in MVPA during PE classes. Therefore, identifying teacher-related factors associated with student in-class MVPA in elementary PE classes is necessary. Purpose: This study focused on students' MVPA during PE in elementary schools in China. The study aimed the following: (a) investigate MVPA of students in PE, teacher characteristics, and teaching behavior; (b) identify differences in teaching behavior through teacher characteristics; and (c) determine the association among teaching behavior, teacher characteristics, and MVPA of students in PE. Methods: A total of 56 teachers from 10 elementary schools in Shanghai, China, provided personal data on teacher characteristics via questionnaire. Data on teaching behaviors were collected from 210 lessons using the System for Observing Fitness Instruction Time (SOFIT), whereas data on MVPA of students were collected using accelerometers. Teacher characteristics, teaching behavior, and students' MVPA time are described using descriptive statistics. Moreover, differences in teaching behavior were tested on the basis of teacher characteristics through multivariate analysis of variance. The associations among teacher characteristics, teaching behavior, and students' MVPA were examined through hierarchical regression analyses. Findings: The teachers' lessons failed to reach the recommended 50% MVPA time in PE. Compared with PE teachers with 6-10 years and more than 10 years of teaching experience, teachers with 1-5 years of teaching experience spent significantly more time managing classes and less time observing their students. Teacher gender was found to be negatively related to students' MVPA, that is, students spent significantly more MVPA time in male- than female-led lessons (B = -3.105). Teaching experience of 6-10 years (B = 2.726) and more than 10 years (B = 3.036) were positively associated with students' MVPA, which indicated that students taught by these two groups of teachers spent significantly more time in MVPA than teachers with 1-5 years of teaching experience. With regard to teaching behavior, promoting (B = 1.207) and observing (B = 1.567) were positively correlated with students' MVPA during PE classes. Regression analysis revealed that 17.3% of the variance for students' MVPA was explained by teacher characteristics and behavior. Conclusion: Results suggested that all teachers should be targeted to increase MVPA level of students in PE, especially female teachers and teachers with 1-5 years teaching experience. PE interventions should target the teaching behavior of promoting and observing to increase students' PA level during PE classes.