A combination of a CH4 supersonic molecular beam and a scanning tunneling microscope (STM) has been applied to analyze the formation processes of a monolayer graphite (MG) on a Pt (111) surface. It is found that a full mono-layer develops only when the surface normal component of the CH₄ kinetic energy exceeds the threshold much greater than the activation barrier height for the CH₄ dissociation on Pt (111). The major cause of the reaction termination below the threshold energy is the enhancement of the activation barrier height due to the reduction of the surface work function, which is resulted from the graphite formation.