It was found that the cationic ring-opening polymerization of tetrahydrofuran (THF) was initiated by carbon black containing CO+ClO4-groups, which had been introduced by the reaction of COCl groups with AgClO4, in the presence of promoters such as epichlorohydrin (ECH) and diketene. It was confirmed that formation of poly(oxytetramethylene), i.e., poly-THF, was propagated from CO+ClO4-groups on carbon black surfaces and effectively grafted onto it. The grafting ratio with ECH as promoter increased to 66.8% as conversion increased. The activation energy of the polymerization initiated by CO+ClO4- groups in the presence of ECH was determined to be 8.1 kcal/mol. Furthermore, poly-ECH, poly(styrene oxide), and poly(propylene oxide) can be grafted onto carbon black surfaces by the ring-opening polymerization of the corresponding alkylene oxides with CO+ClO4- groups on the carbon black as initiator. The polyether-grafted carbon black obtained from the above polymerization produced a stable colloidal dispersion in a good solvent for polyether.