The characteristics of CO emission from HCCI engine operated on Dimethyl Ether have been investigated from experiments and chemical kinetic computations. Normally, the compression ignition of Dimethyl Ether has two stages of combustion, however, in some cases, it has three stages of combustion. The 3rd stage of combustion was mainly composed of CO oxidation reactions and played an important role in CO emission. From the computations, it was confirmed that the dominant path for CO oxidation is the reaction of CO+OH=CO_2+H. The produced H atoms react with O2 molecules and produce OH+O or HO_2 The CO oxidation becomes active as increasing the blanching fraction to OH+O. However, with the addition of CO_2 to the mixture, the branching fraction to HO_2 was increased, resulting in a higher CO emission. Though the addition of CO_2 is effective in suppressing a knocking, it has an inferior effect on CO emission.