The effect of thermal dechlorination treatment of incineration fly ash, which is a heating at 300 – 400 ℃ under reducing condition, on polychlorinated naphthalenes (PCNs) are studied. Fly ash before and after the treatment from five different municipal solid incineration plants were sampled. Contents of PCNs and dioxins are analyzed, and total contents, congener distribution and dioxin-like toxicity were compared before and after treatment. The total contents of PCNs decreased in 2 ashes and increased in 3 ashes. However, highly chlorinated compounds decreased in all ashes, resulting in the decrease of dioxin-like toxicity. When the effects on dioxins and PCNs were compared, thermal dechlorination treatment seemed to have a toxicity reduction effect on PCNs equal to or greater than that on dioxins.