The intelligent tutoring system (ITS) enables students to learn knowledge deductively. However, students often become passive, because the ITS takes the initiative in their learning process. Also their knowledge is often superficial, beacause they can not understand different kinds of knowledge due to their limited experience. This paper presents a virtual learning environment (VLE) for discovery learning. The VLE has been built with virtual reality (VR) technology, and supports the student's discovery learning activity and fosters his/her creativity and adaptability based on a broad range of experience by using the functions of VR such as interactivity, direct manipulation interface, walk-through, the function to change view point freely. Also, the VLE connects the explorative training by means of VR with guided education by the ITS. The student model in VLE evaluates the student's level of understanding and adjusts the training accordingly. We have built an operator training system for the training of control activities of electric power plant using the conception of the VLE. The purposes of this system are the following: to aid students to acquire adequate knowledge and skills, and to aid them to gain confidence and experience through their learning activities. The student model evaluates the student's level of understanding for experiential knowledge connected that of skills in VR with that of knowledge in ITS.