The purpose of this study was to explore the effectiveness of adaptive assessment versus learner control in a multimedia learning system designed to help secondary students learn science. Unlike other systems, this paper presents a workflow of adaptive assessment following instructional materials that better align with learners' cognitive development and ability. The results showed that students made significant improvements after learning with the tutor control learning system. Based on the results of adaptive assessment, the system provided just-in-time personalized instructional materials that better aligned with students' knowledge. Students spent less time reading the instructional materials, which increased the learning efficiency. The findings are discussed in terms of tutor versus learner control, and recommendations are provided for future design and research in the area of multimedia learning system.