The thermal fluctuation and elasticity of dioleoyl-phosphocholine large unilamellar vesicle interacting with pore-forming peptide, melittin, were investigated by neutron spin-echo measurements. The relaxation behavior of the membrane fluctuation with different peptide to lipid molar ratio P/L  can be divided into three regions, resulting from characteristic changes of the effective bending modulus κ ˜   of the membrane which includes the effects of internal dissipation within the membrane. At low P/L  , melittin is adsorbed parallel to the surface of membrane and κ ˜   decreases significantly due to perturbation of hydrocarbon chain packing. At a critical P/L  , melittin forms pores in the membrane and κ ˜   starts to increase slightly due to high pore rigidity. At higher P/L  where the repulsive interpore interaction becomes significant, κ ˜   increases rapidly.