Globular aggregates of carbon nanohorns (CNHs) often contain graphite-like thin sheets (GLSs), and providing different functions to CNHs and GLSs would expand the possible applications of the CNH-GLS aggregates. We show that the GLS edges can be carboxylated selectively by immersing the aggregates in an aqueous solution of H2O2 at room temperature for 1 hour. The presence of carboxyl groups was confirmed by temperature-programmed desorption mass spectroscopy measurements, and their amounts were evaluated using thermogravimetric analysis. The preferential carboxylation of GLSs at their edges was evidenced, after the carboxyl groups were reacted with Pt-ammine complexes, by electron microscopic observation of the Pt atoms at the GLS edges. Since few holes in CNH walls were opened by the short-period H2O2 treatment, there was little carboxylation of CNHs.