In order to identify predominant circulating types of infectious bronchitis(IB) virus in the period of 1999-2004 in Iran, samples(trachea, lung, kidney) from 150 various flocks showing respiratory signs suspected to be related to IB infection were inoculated to SPF eggs. Then allantoic fluids studied by RT-PCR and Nested PCR using group and type specific primers to probing Massachusset, 793/B and D274 types viruses respectively. Out of 150 tested flocks approximately 72% were IBV positive in RT PCR tests. Specific nested PCRs on RT- PCR products of tested samples revealed that 57 flocks (52.7%) had been infected by 793/B type IBV alone. Massachusetts type IBV also could be detected in 18 flocks (16.6%) alone. Thirty three (30.5%) flocks had been infected by 793/B and Massachusetts type IBV in combination. Collectively, there were 90(approximately 83%) flocks that were infected by 793/B type IBV. Our results showed that the predominant circulating serotype of IBVs during the study period was 793/B type. The 274D type did not detected in any flocks during this study. The co-existence of Masachusset and 793/B type’s viruses in the same times in 33 flocks revealed that Masachuset type virus vaccines have not conferred a proper immunity against 793/B type viruses.