Differential item functioning (DIF) occurs when individuals at the same level on the trait(s) or construct(s) being assessed but from different subpopulations have unequal probabilities of attaining a given score on the item (Rojers, 2005). In this study, we measured the Gender Differential Item Functioning in grade seven mathematics test composed of 25 items, which is part of the Cognitive Development Program in Sultanate of Oman. Mental-Haenszel (MH) and Item Characteristic Curve (ICC) Methods was used to detect DIF. The sample included 27329 (13258 male, 13801 female). The analyses showed that ICC was more rigorous (ranged 56% - 80%) than MH (ranged 48% - 52%) in detecting DIF. In addition, the study found significant differences in number of differential items due to sample size. Several recommendations were presented on the basis of the finding.