The　effect　of　ferrocene　additive　on　high　load　knock　of　homogeneous　charge　compression　ignition　(HCCI)　engine　was　investigated　by　an　experiment　carried　out　with　the　fourth　cylinder　of　a　fourcylinder　diesel　engine.　The　experimental　results　indicate　that　ferrocene　can　restrain　the　HCCI　engine　knock　and　extend　the　HCCI　operational　load　range;　at　the　ferrocene　mass　fraction　of　0.00025,　the　knock　is　abated　and　the　load　range　is　enlarged　a　little;　when　the　ferrocene　mass　fraction　increases　to　0.00035　and　0.0005,　the　knock　and　the　load　range　are　improved　a　lot,　but　higher　ferrocene　fraction　resulted　in　more　likely　engine　misfire;　the　optimal　mass　fraction　of　ferrocene　is　0.00035　at　1400　r/min.