The advancement of computer and communication technologies has enabled students to learn across various real-world contexts with supports from the learning system. In the meantime, researchers have emphasized the necessity of providing personalized learning guidance or support by considering individual students’ status and needs in order to improve their learning performance. Based on this perspective, this study proposes a formative assessment-based approach for improving the learning performance of students in a personalized learning environment. An integrated learning diagnosis and formative assessment-based personalized web learning system was developed based on this approach. To evaluate the effectiveness of the proposed approach, an experiment was conducted in an elementary school mathematics course in Thailand. The experimental results showed that (1) the students learning with the proposed system revealed significantly better learning achievement and learning perceptions than those learning with the conventional learning system and (2) students’ learning perceptions of the system had significant relationships with their learning achievement.