Purpose: The purpose of this study was to develop and cross-validate an equation based on ActiGraph accelerometer GT3X output to predict children and youth's energy expenditure (EE) of physical activity (PA). Method: Participants were 367 Chinese children and youth (179 boys and 188 girls, aged 9 to 17 years old) who wore 1 ActiGraph GT3X accelerometer on their right hip during the following tests/activities: resting metabolic rate (RMR), six 5-min treadmill walk/runs (tested at different speeds: 3 km·h[superscript -1], 4 km·h[superscript-1], 5 km·h-[superscript- 1], 6 km·h[superscript-1], 7 km·h[superscript-1], and 8 km·h[superscript-1]),1 broadcast gymnastics, and 2 table-tennis exercises. Participants' oxygen consumption was measured using Cosmed K4b[superscript 2]. The participants were randomly divided into a calibration group ("n" = 331, 90%) and a cross-validation group ("n" = 36, 10%). The calibration group's data were used to determine the relationship between EE and triaxial vector magnitude counts (VM) using the Pearson correlation and to derive the equation using a stepwise multiple regression. In the cross-validation group, differences between measured and predicted EE were evaluated using pairwise "t" tests. Results: VM activity counts had a moderately high correlation with EE ("r" =0.758, "p" < 0.01). An EE prediction equation was developed: EE (kcal·min[superscript- 1]) = 0.00083 × VM+0.073 × weight - 2.01 ("R"[superscript 2] = 0.72, SEE = 1.45 kcal·min [superscript- 1]). According to the cross-validation study results, this equation could predict the EE within the range of known accuracy (i.e., about 20% error). Conclusions: An equation based on ActiGraph accelerometer VM activity counts was derived to predict EE of PA in Chinese children and youth within the range of known accuracy.