beta-Thalassemia is the most common single gene disorder in Iran and more than 25,000 affected individuals have been reported. It has been reported that in patients with beta-thalassemia in the presence of Xmn1 polymorphic site the level of Hb F and (G)gamma: (A)gamma ratio is increased. The prevalence of Xmn1 polymorphic site, (G)gamma: (A)gamma ratio and Hb F in 197 beta-thalassemia major patients from the Kermanshah Province of Iran were studied. The Xmn1 polymorphic site was determined by PCR-RFLP procedure. The levels of (G)gamma and (A)gamma chains were detected by HPLC. The percent of Hb F was determined using electrophoresis method. In beta-thalassemia major patients the frequency of presence Xmn1 was 0.39. The mean of (G)gamma: (A)gamma ratio was found to be 2.5. In the present study it was found that in the presence of Xmn1 polymorphic site (G)gamma percent and (G)gamma: (A)gamma ratio were significantly increased (P = 0.01) and the clinical features such as splenomegaly and bone marrow expansion were significantly improved (P = 0.01). We found that in the presence of Xmn1 polymorphic site on both chromosomes (+/+) the level of Hb F tended to be increased compared to the absence of Xmn1 (-/-). The present investigation has studied the frequency of Xmn1 polymorphic site in beta-thalassemia major patients from Western Iran and has revealed that the presence of this polymorphic site caused a positive influence on Hb F production and the (G)gamma percent which could improve the clinical symptoms of beta-thalassemia patients.