This study aimed al comparing Classical Test Theory and Item Response Theory with respect to the effect of' testees ' characteristics on item characteristics and vice versa. The research method was of the applied. descriptive type. Library research served to exam the issue from theoretical point of' view and the descriptive method was employed to treat the issue from theoretical perspective. fn the practical study, the answer sheets of the candidatesfor the university entrance exam in the field ofphysics-math. and emprial science were studied for their performance on the math and biology sections respectively. First, out of the candidates in the physics-math group, 3, 000 were selected based on systematic sampling. Then several sample groups were selected, 11si11g the appropriate software and the size of the sample to respond lo the research questions. Out of' the candidates in the empirical science group, 1000 were chosen for the purpose of this research. The two research questions in this study were treated theorically, and in each question, the advantages of Item Response Theory over Classical Test Theory were examined. The research questions were also examinedfrom a practical point of view. To analyze data, the statistical methods hosed on Classical Test Model - such as mean. item difficulty. item variance, biserial correlation (rb), and point-biserial correlation (rpb) - were applied. KR-20, frequency distribution. and test score plot were employed to analyze the total tests in this model. To analvze data according lo Item Response Theory. statistical methods such as paired t-test, Pearson Cofficient Correlation, Root Mean Square Error (RMSE). and tests of significant were used. The result from the two methods of data analvsis showed that the differences between the two groups of candidates affected the item indexes in CTT. but they did not affect the item parameters in IRT. The results also showed that the ability parameter was affected by Item chtuacteristics in CTT: however, in IRT. item chamcteristicsfailed to measure the ability parameter of the testees and, like the item parameter, the abilitv parameter the invariant and non-skewed.