The present invention relates to MRI contrasting agent for contrasting cancer cell which contains ultrafine nanoparticles. More particularly, the present invention is directed to a self-assembled ligand composition comprising a ligand A, which is separated at a specific pH range, and a ligand B of which surface charge changes at a specific pH range, MRI contrast agent for contrasting cancer cell comprising said ligand composition and MRI contrasting nanoparticles, and the methods for preparing them.