A novel hydrosol of alkanethiol-modified Au nanoparticles was reported for constructing large-scale ordered two-dimensional (2D) self-assembly for the first time. The degree of long-range order is displayed by both the low-angle transmission electron diffraction and fast Fourier transform pattern of the electron microscope image. We discuss the preparation of stable hydrosol of Au nanoparticles coated with octanethiol and the role of ethanol and octanethiol in the preparation as well as the 2D self-assembly of Au nanoparticles from the hydrosol in reference to the nanoparticle without stabilizers.