All-atom molecular dynamics (MD) simulations are performed to examine the stabilities of a variety of binding configurations of alamethicin, a 20-amino-acid amphipathic peptide, in the bilayers of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) and 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC). The binding free energy of alamethicin is calculated through a combination of MD simulation and the energy-representation theory of solutions, and it is seen that the transmembrane configuration is stable in both membranes. A surface-bound state is also found to be stable due to the balance between the attractive and repulsive interactions of the peptide with lipid and water, and the key role of water is pointed out for the stability in the interfacial region. A difference between the POPC and DMPC systems is noted when the polar C-terminal domain is buried in the hydrophobic region of the membrane. In POPC, the peptide is unfavorably located with that configuration due to the loss of electrostatic interaction between the peptide and lipid.