The effect of sodium sulfide (Na₂S) on the formation of chloribnated aromatics such as polychlorinated biphenyls (PCBs) and chlorobenzenes (CBz) in post combusiton zone (200-500°C) of municipal solid waste incinerator was investigated. When fly ash was heated with mixture of 2% Na₂S at the condition of 300°C and 30min in air with 500ppm hydrogen chloride, total levels of CBz and PCBs decreased by 49% and 22% compared with that of fly ash without addition of Na₂S, respectively. The analysis using the sequential chemical extraction method indicated copper compounds in fly ash changed into copper sulfide. The main inhibiting mechanism was estimated to be the reduction of catalytic activity of fly ash.