Fe3O4　nanomaterials　have　attracted　tremendous　attention　in　the　field　of　magnetic　resonance　imaging　(MRI)　because　of　their　low　cost,　good　biocompatibility,　favorable　superparamagnetic　properties.　This　article　focuses　on　the　controlled　preparation　methods　of　superparamagnetic　Fe3O4　nanoparticles,　and　provides　an　in-depth　discussion　of　the　key　factors　and　their　influence　rules　for　particle　size,　crystallinity　and　magnetic　properties.　The　current　status　of　available　methodologies　and　mechanisms　for　the　assembly　and　surface　modification　of　Fe3O4　nanoparticles　are　highlighted　to　generate　high　performance　and　multifunction.　We　also　systematically　summarize　the　effect　of　particle　size,　morphology　and　surface　properties　on　the　magnetic　and　toxic　properties　of　Fe3O4　nanoparticles.　Finally,　the　future　opportunities　and　challenges　of　Fe3O4　nanomaterials　used　as　MRI　contrast　agents　are　addressed　to　our　understanding.