Solid-electrolyte interphase (SEI) formation at highly ordered pyrolytic graphite (HOPG) surface has been studied with in-situ AFM. The morphology and thickness of both the top particle layer of the SEI and the bottom layer of the SEI that was caused by lithium insertion were investigated. The formation mechanism of the SEI was proposed accordingly. Ex-situ FESEM and EDS were also used to analyze the composition of the electrode after cycling to confirm the proposed mechanism.