Although chemical functionalization at carboxyl groups of nanocarbons has been vigorously investigated and the identities and quantities of the carboxyl groups have been well studied, the location of carboxyl groups had not previously been clarified. Here, we show that site identification of carboxyl groups is possible by using Pt-ammine complex as a stain. After Pt-ammine complexes were mixed with graphenes in ethanol, many Pt-ammine complex clusters with an average size of about 0.6 nm were found to exist at edges of graphene sheets, indicating that the carboxyl groups mainly existed at the graphene edges. These results will make it easier to add functionalities by chemical modifications for various applications of nanotubes and other nanocarbons.