HCCI　engine　is　prone　to　have　a　knock　at　high　loads　limiting　its　power　output.　In　order　to　control　its　knock　at　high　loads,　a　CO2　apparatus　was　installed　to　verify　the　feasibility　of　ingesting　CO2　into　the　intake　to　appease　engine　knock　on　an　engine　bench.　HCCI　combustion　tests　were　conducted　on　the　fourth　cylinder　of　a　modified　4-cylinder　diesel　engine.　The　experimental　results　indicated　that　under　steady　engine　speed　of　1400　r/min　and　fixed　fueling　rate,　high　volume　fraction　of　CO2　can　decrease　the　peak　values　of　HCCI　combustion　cylinder　pressures　and　heat　release　rates　gradually,　increase　causing　combustion　duration,　ease　engine　knock　and　enlarge　the　operating　range　at　high　loads　effectively.　However,　the　probability　of　engine　misfire　increases　with　the　increase　of　the　volume　fraction.　For　the　tradeoff　of　knock　and　misfire,　the　volume　fraction　of　CO2　should　be　about　1%.