The main purposes of this research are to discuss how to establish a Computerize Diagnostic Test System that can be used to identify the wrong concept of a student and is applicable to make a remedial online teach . Finding out a testing procedure to complete the diagnosis test and remedial instruction within a class. The function of this research is to reduce the examination time ,to make effective study. According to the elementary math course of the fourth grade student . We can analyze the constructure of knowledge;to make the Adaptive Diagnostic Test. Computerize Adaptive Diagnostic Test System have the individual and adaptive characteristics. With such characteristics, it can save the time of a test and make it easy to carry a remedial instruction out. Furthermore, a teacher would not spend a lot of time to grade the students’ test papers by using the system. An immediately reply is also available for both the student and the teacher.