β-Thalassemia is common in countries of the Middle East including Iraq with the high prevalence of major and intermedia phenotype. Xmn I polymorphism Gγ -158 (C>T) is the most frequent genetic modifier that implicated in the variation of β-Thal phenotypes. Studies from Iraq on the relevance of this polymorphism to the phenotype of β-Thal are scare, so this study investigated the frequency of the Xmn I polymorphism in β-thal major and intermedia in Iraqi patients and examined correlations with the HbF levels, red blood cell indices, age at diagnosis and number of annual blood transfusion requirements. For this purpose a total of 140 peripheral blood samples (70 from β-thal major patients and 70 intermedia) were enrolled. The presence or absence of the Xmn I polymorphism was analyzed in both groups by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). The results of this study showed that the T allele frequency is (25.7%) in β-thal intermedia (p= 0.0001), as compared to (7.8%) in β-thal major. This polymorphism showed a strong positive influence on HbF production, levels of total hemoglobin, MCV and MCH. The mean age at diagnosis was higher in the presence of the polymorphism while the mean of blood transfusion in the previous year was reduced in both of β-thal major and intermedia. Therefore, the presence of Xmn I polymorphism in the promoter region of the Gγ-globin gene results in ameliorating effects on the phenotype of β-thal in the Iraqi population in Mosul.