Local density approximation within the framework of the density functional theory is applied to calculate the scanning tunnelling microscopy(STM)images of Ar atoms adsorbed on a graphite sheet(Ar/graphite system).It is found that the optimal site of adsorbed Ar atom is at the top of the centre of the carbon hexagon and its equilibrium distance from the graphite surface is about 3.20A.We demonstrate that it is the hybridization of the C 2 p electronic states with the Ar 3 p and 4 s electronic states,which renders Ar atoms visible in the STM experiment.