In this study, we have developed a web-based adaptive testing system using item response theory´s computerized adaptive testing to diagnose underachievers, and to check the evaluation results immediately. Adaptive testing system simple is not the fact that it presents a question to students. It calculates information of a question and presents the question to students. It controls the response of the students under extraction conditions of the next question. It extracts the question which is the most suitable it presents. In this adaptive testing system, you can extract questions according to the level of the students, and adjust the length and the level of the difficulty according to the response of the students.