Mechanochemical (MC) treatment using calcium oxide (CaO) is one of the new technologiesapplicable for decomposition of dioxins contaminating fly ashes. This study was aimed to reveal their decomposition pathways in the MC treatment using 4-chlorobiphenyl (4 CB) as a model compound of co-PCBs, one of the dioxin families. Ion chromatography showed that the MC treatment caused dechlorination of 4 CB in a final ratio of 100%. The finding that the chlorine balance during the MC treatment was accounted for chlorine ion and chlorine contained in the remaining 4 CB indicated that the first step of 4 CB decomposition in the MC treatment was dechlorination. Biphenyl, cyclohexyl benzene, and ter/quaterphenyls were identified as the main products of 4 CB decomposition by a GC-MS analysis, while MC treatment of biphenyl and cyclohexyl benzene yields ter/quaterphenyls and bicyclohexyl, respectively. In addition, MC treatment of p-terphenyl confirmed benzene ring cleavage, i.e., 4-penthyl biphenyl and isopropyl biphenyl were detected as p-terphenyl decomposition products. These results suggested that 4 CB decomposition progresses via dechlorination/hydrogenate-reduction or dechlorination/polymerization, and benzene ring cleavage.