Abstract: This work investigates the effects of diesel oxidation catalyst (DOC) on emissions of HC, CO, NO_x on a diesel bench under the conditions of two steady speeds with various load and five operating modes. In addition, the carbon composition of PM is analyzed by DRI thermal/optical carbon analyzer. It was found that DOC could significantly decrease the emissions of HC、CO, but had no conspicuous effect on NO_x emission. DOC was effective in reducing PM concentration by 25%~40% and the reduction efficiency of nuclear mode was 16%~36%. With the continuous increment of load, the number transformation efficiency of total particle as well as nuclear mode decreases gradually, while number transformation efficiency of accumulation mode increases gradually. Further, DOC is effective in reducing organic carbon (OC) emissions through the reduction of HC, the decrement of OC contributes above 67% to the total PM decrement, while EC contributes only lower 9%.