The electrochemical intercalation and surface morphology of highly oriented pyrolytic graphite(HOPG) electrode in2 mol·L~(-1) HClO_4 were studied by in situ scanning tunneling microscopy(STM) and cyclic voltammetry(CV). Based on the step-height changes observed before and after the intercalation, the effects of different step sites on intercalation are compared. The feasibility,reversibility and speed of intercalation are discussed. The intercalation of ClO_4~- into HOPG can be divided into three types depe...