Membrane area fluctuation of the lipid bilayer has been investigated based upon two-dimensional Voronoi tessellation analysis for the centers of mass of the lipid molecules projected on the bilayer plane. Long-time trajectories of the molecules used in the analysis have been generated by molecular dynamics calculations. The single-molecular area defined by Voronoi polygon showed a broad Gaussian distribution, from which area distribution of the membrane composed of N lipid molecules may satisfactorily be predicted. The fluctuation was found to be caused mainly by thermal motions of the alkyl chains. The number of gauche conformation and alkyl chain length was strongly correlated to the molecular area. Head group motions, however, showed little contribution to the fluctuation. Geometry of Voronoi polygons and the number of nearest neighbor molecules showed rather broad distribution due to the thermal fluctuation. This is in contrast to the structure found in the ripple, gel, and crystal phases. Formation of large pores in the membrane was also investigated.