This study investigated the association between a change in travel mode to school and one-year changes in physical activity (PA) among children in Hong Kong. Data from 677 children aged 7-10 years (56% boys) who participated in the Understanding Children's Activity and Nutrition (UCAN) study were analyzed. During the 2010/11 and 2011/12 school years, the children wore an accelerometer for a week and their parents completed a questionnaire about the children's modes of travel to school and nonschool destinations. Associations between a change in the mode of travel to school and changes in moderate-to-vigorous PA (MVPA) were determined using linear mixed models, adjusting for covariates. Compared with children who consistently used passive travel modes, a change from passive to active travel to school was positively associated with changes in the percentage of time spent in MVPA (b = 1.32, 95% CI = 0.63, 2.02) and MVPA min/day (b = 10.97, 95% CI = 5.26, 16.68) on weekdays. Similar results were found for weekly MVPA. Promoting active travel to school may help to combat age-related decline in PA for some Chinese children. However, maintaining active travel to school may not be sufficient to halt the decreasing trend in MVPA with age.