The functionalization of self-supporting graphene (SSG) which in-situ grows on a flexible graphite sheet is performed by an electrochemical exfoliation in a HCl solution and then a diazonium reaction using p-aminobenzenesulfonic acid. The functionalized SSG is defined as F-SSG. The grafting yield of benzenesulfonic acid groups on F-SSG is up to 7.5%. The conductivity of the F-SSG is 502.7 S/cm. The water contact angle of F-SSG is only 28.6 degrees, which is much smaller than that of SSG, displaying good hydrophilicity.