Single-walled carbon nanohorn (CNH) is a type of nanocarbon that is a single-graphene nanometer-sized pseudo tubule with a diameter of 2-5 nm and a length of 40-50 nm. About 2000 CNHs assemble to form a spherical aggregate with a diameter of about 100 nm. CNHs have low toxicity and their potential medical applications have therefore attracted much research attention. In this report, we show that the cellular uptake of CNHs can be controlled by chemical functionalization of the CNH surface and that the controllability can be enhanced by downsizing the CNH aggregates.