Infectious bronchitis (IB) disease is one of the important respiratory diseases of poultry that causes annually large economic losses in poultry industry of Iran. The aim of this study is molecular characterization of S1 gene of Iranian infectious bronchitis viruse (IBV) belonged to 793/B serotype. The whole S1 gene of three local IBV strains in RT-PCR, showed a band above 1600 base pair (bp) in gel electrophoresis. RFLP analysis using 3 restriction enzymes, HaeIII, HindIII and EcorI, showed 793/B serotype pattern. The S1 gene of these strains were sequenced and compared with 30 reference IBV strains.Identity Plot (IP) of nucleic acids and amino acids sequences were also designed. Moreover, nucleic acids differences in 1659 bp of S1 gene were calculated by distance method: nucleotide: Kimura 2-parameter and finally, the phylogenetic tree of S1 gene sequence strains with the highest validity in branching were designed. Three Iranian strains belonged to 793/B genotype with nucleotide differences of 5.64-6.07 % to UK/793/B as a prototype of 793/B and 26.02-26.16 % to H120 as a vaccinal strain.Regarding to low homology and weak cross protection between 793/B serotype and Massachusetts vaccinal strain,it would be a possible cause for failure in vaccination and
outbreak of IB disease in vaccinated flock of poultry industry.