Objective         To reveal the molecular characteristics and evolutionary selection of hemagglutinin (HA) genes of pandemic H1N1 (pH1N1) viruses isolated from Guangdong from the year of 2012 to 2013．        Methods        The HA genes of Guangdong influenza pH1N1 viruses isolated during 2012 to 2013 were sequenced and compared with global HA genes downloaded from GenBank. The phylogenetic tree and the molecular model of HA genes was conducted respectively by MEGA 6.05 and Chimera v1.8.1 as the selected sites were analyzed by DATAMONKEY.         Results        Compared with HA genes of vaccine strain A/California/07/2009

the HA genetic homologies of Guangdong strains during 2012 to 2013 reached to 98.1% to 98.7%. There were some amino acid substitutions including H155Q (Ca2 epitope)

K180Q/N (Sa epitope)

S202T (Sb epitope)

S220T (Ca1 epitope) and R222K (Ca1 epitope)

where the A151V was located in receptor binding sites (RBS) 140 loop. Residue 202 was revealed to be the positively selected site. Moreover

the HA amino acid mutations of A/Guangdong/64/2013 occurred in V6I

N55S

S101G

A151V

H155Q and V267A．        Conclusion        The mutations in HA genes of Guangdong pH1N1 viruses during 2012 to 2013 lead to variations in antigenic epitopes and RBS loop

which might respectively influence the antigenicity and the receptor-binding. Mutation AA202 with positive selection means to be borne the great immunological pressure on it. Epitope Ca2 and RBS mutation sites in strain A/Guangdong/ 64/2013 could contribute to the dominant strain in the following influenza season．