The addition of deceleration system as reducer of CO emission in gasoline-motor vehicle. This research is aimed to investigate the impact of addition of deceleration system on the CO reduction emitted from gasoline-motor vehicle. It uses experimental research by comparing CO emission both before and after using deceleration system. The experiment was conducted in Automotive Laboratory the State University of malang. The descriptive analysis is used for picturing the overall comparison and t-test is used for describing the differences. The result shows that there is sharp CO reduction from 4.615% of average to 1.883% following the t-test result that shows significant difference at p 0.000. It can be concluded that the addition of deceleration system can reduce the CO emission from gasolinemotor vehicle.