A　facile　method　to　synthesize　ultrasmall-sized　super　magnetic　iron　Oxide　nanoparticles　with　good　monodispersity　and　high　relaxivity　is　desired　for　magnetic　resonance　imaging　(MRI)　technology.　Herein,　we　have　developed　a　one-step　method　to　direct　the　formation　of　superparamagnetic　iron　oxide　nanoparticle　(uBSPIO)　using　albumin　under　mild　conditions.　The　resulting　uBSPIO　possess　ultrasmall　size　(4.78　+/-　0.55　nm)　and　super　high　MR　relaxivity　(444.56　+/-　8.82　mM(-1)　s(-1)).　After　grafted　by　the　luteinizing　hormone　release　hormone　peptide　(LHRH),　the　uBSPIO　could　targeted　and　accumulated　in　the　tumor　site.　Finally,　the　uBSPIOs　had　good　stability　and　did　not　induce　cytotoxicity　in　vitro　or　major　organ　toxicity　in　vivo.　The　uBSPIOs　are　promising　contrast　agents　for　MRI.