The main purpose of this study was to compare the objectively measured physical activity (PA) and the motivation process between adolescents with (n = 25) and without (n = 75) autism spectrum disorders (ASD) in inclusive physical education (PE); and assess the associations of the PA levels to a sequence of motivational processes. Independent t-tests revealed significant PA and motivational process differences between adolescents with and without ASD. External regulation was positively correlated with the percentage of time that adolescents with ASD spent in moderate PA (r[subscript 25] = 0.58, p less than 0.01) and moderate-to-vigorous PA (r[subscript 25] = 0.50, p less than 0.05), and this extrinsic motive was associated with their needs of being attached or related in the class (r[subscript 25] = 0.53, p less than 0.01). No significant associations of PA in PE on the motivational sequences of adolescents without ASD were observed. It is concluded that adolescents with ASD had less PA levels in PE and lower motives toward PE than adolescents without ASD, and external regulation was important in facilitating PA participation in adolescents with ASD. (Contains 6 tables.)