One of the wastes generated by PT Beton Elemenindo Perkasa is polystyrene foam waste. Processing of this type of waste is still done with open burning so it has an impact on health and the environmental quality degradation. One of the polystyrene foam waste processing technologies is by constructing a combustion furnace equipped with carbon filter. Activated carbon is one of the air filter media that can absorb harmful gases from the combustion process. The purpose of this research is to identify the effectiveness of carbon monoxide (CO) concentration reduction on active carbon contact system using variation of particle size. This research is a kind of an experimental study involving post test with control design. The number of samples was calculated based on the number of treatments and the number of repetitions in the study. This research used 2 kinds of treatment, including 20 mesh and 30 mesh in 9 repetitions. Independent T-Test statistical analysis showed a significant difference (p-value = 0.001) between the variation of particle size of activated carbon and the CO parameters with the average of the most effective reduction percentage on particle-sized active carbon of 30 mesh was 77.95%.