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This study examined the prospective associations of reallocating time spent in different types of sedentary behavior, physical activity, and sleep with body mass index (BMI) in children using isotemporal substitution analysis.Chinese children in grades 1–3 were recruited to a cohort study in 2009 and were followed up over a 2-yr period. Reports were gathered from the parents on children’s sedentary behavior, sleep duration, and sociodemographic variables. The reported sedentary behavior types were then grouped into three categories: screen time (e.g., watching TV), academic-related activities (e.g., doing homework), and other sedentary behaviors (e.g., sitting and talking). Moderate-to-vigorous physical activity (MVPA) and light-intensity physical activity were assessed by ActiGraph accelerometry (ActiGraph, Pensacola, FL). Isotemporal substitution models were performed to examine the effects of time allocation on BMI.A total of 672 children (359 boys, mean age at recruitment = 7.6 yr) who had provided valid accelerometer data for at least one assessment time point were included in the analysis. Controlling for covariates and total behavior time, isotemporal substitution models indicated that the displacement of 30 min·d−1 of other sedentary behaviors with equal amounts of screen time (B = 0.12; 95% confidence interval, 0.04–0.20) or academic-related activities (B = 0.13; 95% confidence interval, 0.04–0.21) was associated with higher BMI. Reallocating 30 min·d−1 of MVPA with each of the sedentary behavior variables resulted in increased BMI.The substitution of screen time or academic-related activities with other sedentary behaviors or MVPA was associated with lower BMI in Chinese children.