HA facilitates viral entry through binding to the host surface sialic acid residues [21] . Accordingly, if HA is blocked at its sialic acid binding site by a small molecule, the viral entry process will be stopped and the penetration of viruses into host cell prevented. In comparison with NA inhibitors, the HA inhibitors were usually more effective in inhibiting influenza virus. For all the HA subtypes (H1-H16) so far identified [22] , the HA1 subtype from the recent pandemic H1N1/09 virus was taken as the target for constituent screening and drug design [23] .