This study aims at identifying the effect of multiple-choice test items' difficulty degree on the reliability coefficient and the standard error of measurement depending on the item response theory IRT. To achieve the objectives of the study, (WinGen3) software was used to generate the IRT parameters (difficulty, discrimination, guessing) for four forms of the test. Each form consisted of (30) items with different difficulty coefficients averages (-0.24, 0.24, 0.42, 0.93). The resulting items parameters were utilized to generate the ability and responses of (3000) examinees based on the three-parameter model. These data were converted into a readable file using the (SPSS) and the (BILOG-MG3) software. Then the reliability coefficients for the four test forms, the items parameters, and the items information function were calculated, and dependence on the information function values to calculate the standard error of measurement for each item. The results of the study showed that there are statistically significant differences at the level of significance ([alpha] = 0.05) between the averages of the values of the standard error of measurement attributed to the difference in the difficulty degree of the items in favor of the test with the higher difficulty coefficient. The results also found that there are apparent differences between the test reliability parameters attributed to the difficulty degree of the test according to the three-parameter model in favor of the form with the average difficulty degree.