As Internet usage becomes more popular over the world, e-learning system, e.g., online learning, employee training courses, e-books, etc. has been accepted globally. How to provide the optimum content for individualized learning becomes an important issue for e-learning system. In this work, a Computer-Assisted Learning Expert System (CAL-ES), based upon expert system methodology, is proposed to provide individualized teaching materials for learners in accordance with their learning aptitudes and evaluation results. According to the cycle of knowledge management in CAL system, which is similar to the cycle of general knowledge management, four mechanisms, i.e. Knowledge Representation, Knowledge Acquisition, Knowledge Organizer, and Knowledge Miner, are designed in CAL-ES. The Object Oriented Course Model (OOCM) is used as the presentation of knowledge in designing CAL system, and is used in CAL-ES. Knowledge Acquisition technology is applied to acquire knowledge from experts, including the knowledge for constructing Teaching Object, building Teaching Materials, and managing Teaching Resources. Knowledge Organizer is a directory service for efficiently managing teaching resources. Finally, Knowledge Miner is used to find implicit knowledge from historical records of the system using Data Mining technologies. These four mechanisms in CAL-ES provide complete knowledge system capability for a CAL system. In the last three years, we have been working on building an integrated CAL platform in the Virtual Mathematical High School project, and the prototype of CAL-ES will be introduced in this work.