Objectives: The objectives of this study were to observe the developmental trajectories of motivation types among young children from 8 to 12 years using a more comprehensive scale of physical education motivation. We also tested the relations between these trajectories and objective physical activity during this period. Design: Students in grades 5–7 (n=1202; 51.2% boys) were recruited from 17 elementary schools. Three cohorts completed the motivation questionnaire four times and objective physical activity was measured up to four times over a two years school period. Method: Measurement invariance of the scale was tested across cohorts and occasions. Multiple group multiple cohort growth models were estimated to determine motivation types trajectories. Regression models were then built to predict children's slope of MVPA during this period. Results: We provided strong measurement invariance to a new and more comprehensive scale of PE motivation. Latent growth curve modeling indicated trajectories that decrease on average for all forms of motivations at this early age. Results also revealed some relations between motivation's scores and objective physical activity trajectories, especially with autonomous motivation. Conclusions: Our study revealed the earliest decline of motivation towards physical education to have ever been highlighted in elementary school children. Relations between trajectory of intrinsic stimulation and PA behavior permitted us to highlight the possible role of autonomous motivation in minimizing the decline of children's PA behavior during PE lesson.