An　efficient　two-step　quantum　key　distribution　(QKD)　protocol　with　orthogonal　product　states　in　the　n　circle　times　n(n　＞=　3)　Hilbert　space　is　presented.　In　this　protocol,　the　particles　in　the　orthogonal　product　states　form　two　particles　sequences.　The　sender,　Alice,　first　sends　one　sequence　to　the　receiver,　Bob.　After　Bob　receives　the　first　particle　sequence,　Alice　and　Bob　check　eavesdropping　by　measuring　a　fraction　of　particles　randomly　chosen.　After　ensuring　the　security　of　the　quantum　channel,　Alice　sends　the　other　particle　sequence　to　Bob.　By　making　an　orthogonal　measurement　on　the　two　particle　sequences,　Bob　can　obtain　the　information　of　the　orthogonal　product　states　sent　by　Alice.　This　protocol　has　many　distinct　features　such　as　great　capacity,　high　efficiency　in　that　it　uses　all　orthogonal　product　states　in　distributing　the　key　except　those　chosen　for　checking　eavesdroppers.