Background: Attention-deficit hyperactivity disorder (ADHD) is a risk for learning. Because ADHD commonly includes behaviours of inattention and behaviours of hyperactivity/impulsivity, how the two types of behaviours independently affect children's academic competence remains poorly understood. Aims: To investigate the impact of behaviours of inattention and behaviours of hyperactivity/impulsivity on Chinese students' academic competence. Samples: Parents of 167 preschoolers (Cohort 1), parents of 313 first graders (Cohort 2), and 1,003 high school students (Cohort 3). Methods: The ADHD-RS-IV Preschool version (Cohort 1), ADHD-RS-IV Home version (Cohort 2), and BASC-SRP (Cohort 3) were used to measure behaviours of inattention and hyperactivity/impulsivity. Academic competence was operationalized as school readiness (Cohort 1), math and language arts scores at two time points provided by school (Cohort 2), and self-reported academic performance (Cohort 3). Multiple regressions were performed to investigate the relationship between academic performance and behaviours of inattention alone (Step 1), and behaviours of hyperactivity/impulsivity alone (Step 2), and behaviours of inattention together with behaviours of hyperactivity/impulsivity (Step 3). Results: For each cohort, both types of behaviours were negatively correlated with academic competence. However, regression analyses showed that in Step 3, behaviours of hyperactivity/impulsivity either failed to predict academic competence or predicted better academic competence. Overall, behaviours of inattention alone accounted for a similar amount of variance in academic competence as did behaviours of inattention and hyperactivity/impulsivity combined. Conclusions: Behaviours of inattention presented a risk for academic competence but the effect of behaviours of hyperactivity/impulsivity varied. Implications for instructional strategies for behaviours of inattention were discussed.