Interferon gamma (IFN-g) is a cytokine produced by activated T lymphocytes and natural killer cells. It plays an important role in innate and adaptive immunity. IFN-g gene is polymorphic and its haplotype has been associated with resistance to nematode infestations which are the most important parasites of domestic sheep worldwide. To evaluate IFN-g polymorphism in Iranian Shaul sheep population, 136 blood samples were collected. Genomic DNA were extracted and amplified by PCR using specific primers for exon 3 of IFN-g gene. After gene amplification, SSCP and Heteroduplex mobility analysis carried out on polyacrylamid gel. Five unique SSCP patterns were detected in this population which 3 of them represent 3 alleles. Although heteroduplex mobility analysis demonstrated that all samples were homozygous, SSCP could reveal heterozygosity at frequency of 40. 4% in the studied population. Data obtained from the present study have revealed that IFN-g gene was a highly polymorphic in Iranian Shaul ecotype. This will be useful to study host-pathogen interaction and to evaluate association between resistance or susceptibility to disease with IFN-g in this ecotype.