This study examines how academic effort changes over time and how motivational variables such as self-efficacy, self-determination, and achievement goals (mastery approach, performance approach, and performance avoidance) affect these changes. In this study, a multi-level growth model was used for data from the Korean Education Longitudinal Study 2013 (KELS 2013) for students from the sixth grade of elementary school to the second grade of high school. We specifically investigated the time-independent and time-dependent effects of motivational variables on academic effort. As a result of the analysis, it was first found that students’ academic effort displayed a quadratic function pattern; that is, academic effort decreased with age and the rate of change also decreased over time. Second, self-efficacy, self-determination, and achievement goals showed significant time-independent effects on academic effort, whereas self-determination and performance approach goals showed significant time-dependent effects. Based on the main findings of the study, the longitudinal change of academic effort and the implications of the influence of motivation are discussed.