Paper-and-pencil test uses Classical Test Theory and its test-takers participate the test together. Oftentimes, the test presents issues, including the inability to determine the ability levels of test-takers and questions that are excessively easy or difficult. On the other hand, a Computerized Adaptive Testing (CAT) test, which uses Item Response Theory (IRT), can efficiently determine the ability levels of test-takers in a short time. This paper aims to analyze the difficulty levels of questions of CAT test and the ability levels of test-takers. It examines CAT test based on the following. First, this paper gives a literature review on and an overview of CAT system. This is followed by a description of CAT simulations and the test effectiveness of CAT. Finally, this paper suggests topics for future research in the field of CAT.