The energy band diagram of n-Si in aqueous 2M KOH is constructed by the measured open-circuit potential and the flatband voltage. A photocurrent susceptivity method was proposed to determine the flatband voltage. A flatband voltage of -1.04 V/SCE (saturated calomel electrode) for the n-Si in 2M KOH was determined by this new method. The energy band diagram of the p-Si in the same electrolyte is also constructed. Accumulation of carriers at the n-Si/KOH interface is predicted by the band diagram and confirmed by the ohmic-contact-like I-V curve. Depletion of carriers at p-Si/KOH interface is predicted by the band diagram and confirmed by the rectifying I-V characteristics. The passivation of Si under anodic bias is attributed to the formation of an oxide film. The competition between the oxidation rate and the diffusion rate of oxidation product determines whether the anodic oxidation is an etching or passivation process. The etching oxidation is assumed to be dominated by the electrochemical reaction. The transport of these carriers which participate in the electrochemical reaction can be explained by the energy band diagram. All etching or passivation phenomena are consistent with the prediction from the band diagram viewpoint. The etch stop for heavily doped Si is attributed to the enhanced growth rate of oxide film under high carrier concentration.