The　research　on　instantaneous　center　of　human　knee　joint　is　one　of　more　important　research　areas　of　knee　joint　biomechanics　research.　In　this　paper,　NDI　OPTOTRAK　optical　motion　capturing　system　was　adopted　to　measure　the　movement　information　of　knee　joint　of　one　adult　male.　A　simple　and　efficient　method　about　calculating　instantaneous　center　of　human　knee　joint　was　provided.　To　avoid　experiment　error　caused　by　improper　operation,　Planar　fitting　and　cubic　polynomial　fitting　was　applied　in　processing　data.　The　result　provide　reference　for　assembling　and　designing　clinical　prosthetic　knee.