Hexadecane and heptadecane were found to form two-dimensional (2D) crystals on a reconstructed Au(100) surface in their neat liquids by in situ scanning tunneling microscopy. The structures of the 2D crystals of hexadecane and heptadecane were very similar to each other except for the molecular length. The molecular axis was parallel to the surface plane and was aligned in the [011] direction. The molecular rows ran in the [011] direction, i.e., perpendicular to the molecular axis. There was a height variation among the molecules which was attributed to the corrugation of the reconstructed Au(100) surface. The molecular axis seemed to orient parallel to the corrugation of the reconstructed Au(100) surface.