Membrane fusion plays an important role in many fundamental biological processes. The infection of enveloped virus such as Influenza virus, membrane fusion between host cell membrane and viral envelope is a key point of these infection processes. Hemagglutinin (HA) is the enveloped glycoprotein of influenza virus which mediates the fusion activity. HA causes the fusion of the viral envelope with the endosomal membrane to enable the passage of the viral genome into the host cytoplasm. The mildly acidic pH in the mature endosome induces the transition of the HA structure into fusogenic state.The first 20 residues of the HA2 domain in HA have been identified as the membrane-fusion peptide, which is the only portion to insert into the target membrane at an oblique angle and thereby triggers membrane fusion. All four membrane fusion peptide analogs carried out in this study were based on the sequence of the E5 fusion peptide analog. The substitution of one proline for either Gly12, Gly13 and for both Gly12 and Gly13 are named G12P, G13P, and monoP, respectively. According to the experimental results, the strict kink which is composed of one proline shows restriction on the rotation of the COOH-terminal residues, thus the little difference in fusion activity are observed in monoP analogs at various pH. The kink composed of one or two Gly is more flexible than the kink composed of only Pro. A flexible kink may allow a greater rotation of COOH-terminal residues, thus the negative charged residues in this region could rotate to against the phophorus group of membranes; the strict kink in monoP restricts the rotation of COOH-terminal residues. The COOH-terminal modification of HGGH shows little influence at its fusion activity. This may indicate that even the COOH-terminal helix exists at neutral pH, the rotation of residues in COOH-terminus are not suitable for fusion activity.