The statistical properties of the bilayer membranes of diparmitoylphosphatidylcholine (DPPC) in the gel and liquid-crystal phases were studied by Monte Carlo (MC) simulation using potential functions of the Lennard-Jones, the simple Coulomb, and the bond torsion. The simulation was undertaken on a two-dimensional periodic condition imposed on the bilayer model consisting of faithfully described molecules. The structure and ordering of the model bilayers accorded well with experiments, and the segment order parameters were in agreement with those of the nuclear magnetic resonance (NMR) experiments. The two kinds of lipid chains of DPPC do not equivalently behave in the bilayers, and chain 2 has lower ordering than chain 1. The order parameters of the first eight segments of chain 2 in the liquid-crystal model are particularly small and are roughly constant. From electron density analysis, it has been observed that the liquid-crystal bilayer has about one excess water molecule per one lipid molecule in comparison with the gel bilayer. The energy difference between the two bilayer models, taking account of the water contribution, is consistent with the latent heat of the phase transition.