The　effects　of　methanol　and　ethanol　additives　on　homogeneous　charge　compression　ignition　(HCCI)　engine　knock　were　investigated　by　add　small　quantity　of　methanol　and　ethanol　to　the　base　fuel　(PRF90).　Six　test　fuels　were　confected　with　the　volume　fractions　of　1%,　3%　and　5%　containing　methanol　and　ethanol,　respectively,　HCCI　combustion　tests　were　conducted　on　the　fourth　cylinder　of　a　modified　4-cylinder　diesel　engine.　The　experimental　results　showed　that,　under　steady　engine　speed　of　1400　r/min　and　at　the　same　fuel　injection　quantity　per　cycle,　with　the　increase　of　the　volume　fractions　of　methanol　and　ethanol　added,　the　peak　values　of　HCCI　combustion　cylinder　pressures　and　heat　release　rates　decreased　gradually,　causing　combustion　duration　increased,　knock　effectively　controlled,　and　load　range　enlarged.　At　the　small　volume　fraction,　methanol　is　better　than　ethanol　in　controlling　engine　knock,　and　methanol　is　much　better　than　ethanol　with　the　increase　of　the　volume　fraction.　However,　the　misfire　limits　for　the　fuels　containing　methanol　and　ethanol　are　narrower　than　that　for　the　base　fuel,　resulting　in　more　possibility　of　engine　misfire.　For　the　tradeoff　of　knock　and　misfire,　the　volume　fraction　of　methanol　or　ethanol　in　the　base　fuel　needs　to　be　about　1%.