Objective. The purpose of this study was to examine the psychometric properties of the self-efficacy for healthy eating and physical activity measure (SE-HEPA) for preadolescents. Method. The reliability of the measure was examined to determine if the internal consistency of the measure was adequate (i.e., [alpha]s greater than 0.70). Next, in an effort to determine if a two-factor model was a better fit than a one-factor model, as hypothesized, an exploratory factor analysis in a confirmatory factor analysis framework was used to determine model fit. Additionally, the criterion-related validity of the measure was evaluated by conducting correlational analyses to determine if SE-HEPA scores were associated with preadolescent body mass index. Results. Consistent with the hypotheses, Cronbach's alphas indicated good reliability for the measure (i.e., [alpha]s greater than 0.70) and factor analyses indicated the two-factor model provided a better fit than the one-factor model. Additionally, correlations revealed a significant relationship between the two factors (i.e., healthy eating, physical activity) and preadolescent body mass index. Conclusions. The SE-HEPA will allow researchers and clinicians to better understand self-efficacy for behaviors important to weight loss among preadolescents. This could, in turn, inform future efforts aimed at increasing behaviors that promote healthy weight status among this population within the context of family-based weight loss programs. (Contains 4 tables.)