Density functional calculations are used to investigate the mechanism of the distinctly different behaviors of Pd and Pt coating on graphene. Geometric and electronic structural analysis indicates that the electrons on the dz2 orbital of Pd may transfer to the dxz+dyz orbital of Pd by the aid of the up electrons of the graphene. This charge-transfer mechanism enhances the interactions between the Pd coating and the graphene substrate, driving the Pd coating to grow on the graphene, while Pt does not have this behavior. Metal self-assembly hinders Pt atoms to cover the graphene.