p-n junction is a fundamental building block in modern electronic circuits. We report graphene p-n junctions formed by a one-step thickness-dependent surface treatment of mono-/bilayer graphene steps. The junction electronic properties are systemically studied by means of Kelvin probe force microscopy (KPFM) and transport measurements. Because of the dissimilar modifications to graphene electronic properties, the junctions behave distinctly, i.e., two-component resistance-like for organic charge transfer doping and Shottky-junction-like for covalent doping. By exploring the spatially potential distribution, we clarify the potential profiles as well as the transport attributes across the graphene p-n junction interface under lateral bias and electrical gating. Our results not only unveil the detailed properties of graphene p-n junction interface, but also gain an insight into its practical applications in nanoelectronics.