Background and Objective: B-thalassemia is an autosomal recessive disease characterized by reduction or complete absence of b-globin gene expression. It has been estimated that more than 2,000,000 carriers as well as 20,000 patients affected with b-thalassemia are living in Iran, a country with more than 70 million population and great ethnic diversity. In this study we aimed to find out the b-globin gene frequency and determine the spectrum of b-globin gene mutations in Zanjan province (northwest region) of Iran. Materials and Methods: 5527 individuals who were referred for pre-marriage tests to Zanjan clinic as well as 27 thalassemia patients were studied. Altogether one hundred and five chromosomes from 78 unrelated B-thalassemia patients or carriers were examined for b-globin gene mutations by ARMS-PCR and direct gene sequencing. Based on the previous information on common mutations in Mediterranean populations 24 sites were analyzed. Results: It was found that the b-thalassemia frequency is 1.2% for Zanjan region. Using the above techniques, the mutations for 90/105 (86.7 %) of b-thalassemia chromosomes (13 different mutations) were identified. Fifty eight percent of the mutations were of common “Mediterranean” type. Of which, IVS-I 110 mutation showed the highest frequency (29.5%) followed by IVS-II-1 (13.3%), IVS-I-1 (12.4%) and IVS-I-6 (2.9%). 10.5% of mutations were of common Asian Indian mutations (Fr 8/9, 6.7% and IVS-I-5, 3.8%) respectively. CD5 and CD30 and CD36-37 mutations accounted for 13.3% of the mutations. (5.7%, 0.95% and 6.7% respectively) Mutations in 14 chromosomes (13.3%) remained uncharacterized. Conclusion: These data suggests that the spectrum of mutations in Zanjan province differs from those reported from other parts of Iran, but Mediterranean type of mutations are more frequent in Zanjan region. Therefore, in order to save the time and cost, it is recommended that for prenatal diagnosis of thalassemia in Zanjan province analysis of Mediterranean mutations should be considered as a front line screening strategy.