This study aimed to investigate differential functioning items and distractors of a criterion referenced science test for sixth grade. The test is constructed according to item response theory (IRT). To achieve this goal, the researcher designed a test constitutes of (75) multiple-choice items. The test was applied at the end of the school year (20152016-), on a sample of (520) 6th grade students, in Jeddah, Saudi Arabia, (418) of which are from public schools, and (102) of which are from

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... e schools. The results of the statistical program (BILOG-MG3) verified that 61 test items fit the three parameters model, 16 of which presented item differential functioning (IDF). In addition, chi square test (χ2) presented distractors differential functioning (DDF) in (6) of these items, and (75%) of the arbitrators agree on this matter. The study recommendation indicates to the importance of checking items distractors differential functioning, to check if it was the reason of item differential functioning.