During　the　first　cycle　of　lithium-ion　batteries,　a　passivation　layer　is　formed　on　the　surface　of　electrode　due　to　electrolyte　decomposition,　which　is　called　solid　electrolyte　interface　(SEI).　In　this　study,　the　evolution　of　LiFePO4　cathode　surface　and　components　of　SEI　layer　are　investigated　in　different　states　by　high-resolution　transmission　electron　microscopy　and　X-ray　photoelectron　spectroscopy.　The　results　suggest　that　SEI　layer　is　mainly　formed　during　the　first　cycle　and　remains　stable　in　the　following　cycles.　Furthermore,　SEI　layer　can　be　divided　into　two　parts,　and　components　of　these　two　parts　are　different.　The　outer　layer　that　is　near　to　electrolyte　is　composed　of　organic　components,　and　the　inner　layer　that　is　near　to　active　material　is　composed　of　inorganic　components.　Taken　together,　the　study　reveals　SEI　layer　formation　process　on　LiFePO4　cathode　surface,　which　is　of　significance　for　understanding　the　influence　of　SEI　layer　on　electrochemical　performance.