To modify the surface of carbon fiber, the anionic grafting of polyesters from potassium carboxylate (COOK) groups on the surface was investigated. The COOK groups were introduced onto the surface of carbon fiber by the reaction of carboxyl groups (introduced by the oxidation of the surface with nitric acid) with potassium hydroxide. Untreated carbon fiber has no ability to initiate the anionic ring-opening copolymerization of epoxides with cyclic acid anhydrides. On the contrary, the anionic ring-opening copolymerization of epoxides—such as styrene oxide (SO), chloromethyloxirane (ECH), and glycidyl phenyl ether (GPE)—with cyclic acid anhydrides—such as maleic anhydride (MAn), succinic anhydride (SAn), and phthalic anhydride (PAn)—was found to be initiated by the COOK groups on the surface. In the polymerizations, alternating copolymers (i.e., polyesters) were effectively grafted from the carbon fiber surfaces depending on the propagation of the polymer from the surface COOK groups: for example, when the copolymerization of ECH with PAn was carried out at 120 °C by use of COOK groups on carbon fiber as catalyst, the percentage of grafting of the polyester reached as much as 40%. The intrinsic viscosity of ungrafted polyester obtained from this polymerization was determined to be 6 cm3/g (chloroform, 30.0 °C). The initiating ability of carbon fiber increased with an increase in the content of COOK groups. Furthermore, the addition of 18-crown-6 was found to be effective for the increase of the copolymerization rate and for the percentage of grafting of polyester on the surface.