In this work, we report atomic and electronic studies of graphitic carbon nitride (g-C3N4) monolayers on a highly-ordered pyrolytic graphite (HOPG) by using atomic force microscope (AFM), kelvin probe force microscope (KPFM) and low temperature scanning tunneling microscope (LT-STM). The g-C3N4 is the most stable allotrope of carbon-nitrides and a very promising candidate for metal-free coordination chemistry and heterogeneous catalyst. Also it is applicable photocatalytic hydrogen production and fuel cell. After successful spin-coating of the g-C3N4 monolayers on HOPG, we found the inter-molecular interactions between the flakes or the atomic registries of adsorption on the substrate. This coating mechanism and atomic/electronic properties of g-C3N4 on HOPG will be discussed in detail.