Focus　on　the　accuracy　problem　in　machining　process　of　SK21　NC　cam　grinder,　the　precision-machining　restriction　condition　equation　and　the　calculation　method　of　precision　NC　instruction　are　deduced.　The　reversible　calculation　model　from　NC　instruction　to　the　cam＇s　outline　is　given　and　a　software　package　of　error　compensation　is　developed.　Many　error　parameters　which　include　the　abrasion　of　the　grinding　wheel,　load　distortion　of　the　grinding　wheel　axis,　rotation　precision　of　the　cam　axis　and　movement　precision　of　the　worktable　are　measured　through　experiments.　The　finally　grinding　experiments　results　show　that　the　cam＇s　machining　accuracy　can　be　improved　more　than　50%　by　the　error　compensation　method　in　this　paper.