ABSTRACTThis study aims to determine the effect of muffler design on emissions emitted by gasoline engine smoke and make efforts to reduce it by modifying mufflers. Exhaust gas emissions on gasoline-fueled engines contain pollutants such as carbon monoxide (Co) gas. The scheme that is implemented in reducing gas Co, is by comparing the results of the emission test with 3 muffler diameter variables. The research method used is descriptive analysis of experimental testing directly on samples with a standard diameter (22 mm) muffler and two mufflers that have been modified with modified the diameter is 18 mm and 27 mm. Data analysis using Microsoft Office Excel software in the form of tables and statistical analysis The results showed that the oxidation catalyzing process with the addition of air treatment on modified mild steel / MS material mufflers Ø 10 mm can reduce HC pollutants by 37.74%, CO by 23.18%, temperature rise of 28.92%, MS Ø 15 mm HC of 31.45% and CO of 27.47%, increase in temperature of 31.33%. Stainless steel / SS Ø 10 mm HC of 32.35%, CO of 28.76%, temperature rise of 10, 84%, SS Ø 15 mm HC of 37.74%, CO of 42.92%, and temperature increase of 14.46%