With the development in computing technology, item response theory (IRT) develops rapidly, and has become a user friendly application in psychometrics world. Limitation in classical theory is one aspect that encourages the use of IRT. In this study, the basic concept of IRT will be discussed. In addition, it will briefly review the ability parameter estimation, particularly maximum likelihood estimation (MLE) and expected a posteriori (EAP). This review aims to describe the fundamental understanding of IRT, MLE and EAP which likely facilitates evaluators in the psychometrics to recognize the characteristics of test participants.