Abstract: Objective To characterize HA1 gene of influenza v irus subtype H1N1 circulated from 2000 to 2006 in Liaoning local area.Methods Viral RNA was extracted and transcr ibed into cDNA by reverse transcriptase and amplified by PCR.The product of PCR was purified and sequenced by ABI 3100 avant.The sequence data were analyzed with epidemic records.Results (1)The 21 sequences including those from genbank(except the other earlier 2 sequences)can be roug hlydivided into two majo r distinct lineages before and after the year 2000.(2)Four recent Liaoning strain sequences in 2005~2006 have a variation about 3% compared with the other 5 strains of a few years ago.The nucleic acid and amino acid sequence data of HA1 domain showed that there were four important mutant positions,they were 187 W>R,188 Y>F,209 Q>K,249 V> Arespectively.Conclusion The HA1 domain of HA gene of influenza virus(H1N1)isolated from 2005~2006 showed mutation,and the mutated viruses were becoming the dominant circulating strain in Liaoning local area,and showed amino acid sequence difference compared to A/New Caledonia/20/1999,the vaccine components pronounced by WHO from 1999 to 2006,which suggested that further sur veillance should be conducted to monitor the virus mutation in circulation.