We apply first-principles total-energy electronic structure calculations within the density functional theory (DFT) in the local density approximation (LDA) to calculate the scanning tunneling microscopy (STM) images of argon monolayer adsorbed on a monolayer graphite surface. The results are consistent with the experiments. It is demonstrated that the hybridization of C 2p electronic states (pi-electronic states) with Ar 3p and 4s electronic states is the main origin of the Fermi-level local density of state (LDOS), which makes Ar atoms visible in the STM experiment. (C) 2003 Elsevier Science B.V. All rights reserved.