Early　diagnosis　of　cancer　has　become　one　of　the　effective　ways　to　prevent　and　treat　cancer.　At　present,　magnetic　resonance　imaging　(MRI)　diagnosis　based　on　nanosized　iron　oxide　with　no　toxicity　and　excellent　biocompatibility　has　attracted　much　attention.　But　the　poor　stability　of　magnetic　fluid　(MF)　is　becoming　more　and　more　prominent.　In　view　of　the　poor　stability　of　MF,　the　dodecanethiol-poly(methacrylic　acid)　(DDT-PMAA)　as　an　outer　shell　was　prepared　for　the　MFs　(Fe3O4@DDT-PMAA　MF)　to　improve　the　stability.　We　also　compared　the　amount　of　reactants,　the　ratio　of　reactants,　and　the　reaction　temperature,　and　finally　screened　out　the　best　reaction　conditions,　so　as　to　get　more　stable　products.　The　stability　of　the　system　was　studied　by　visual　observation,　stability　index　(I),　and　magnetic　weight　change.　The　results　demonstrated　that　Fe3O4@DDT-PMAA　can　be　stable　for　more　than　60　days.　The　T　1　mapping　image　showed　that　the　longitudinal　relaxivity　(r　1)　value　was　6.9　mM-1　s-1　(1.5　T,　room　temperature),　which　was　higher　than　the　commercial　contrast　agent　SHU-555　(r　1　=　2.9　mM-1　s-1),　but　the　transverse　relaxivity　(r　2)　value　accounted　for　64.48%　of　the　commercial　contrast　agent　Feridex　(r　2　=　67.8　mM-1　s-1,　1.5　T,　room　temperature)　because　the　outermost　layer　was　encapsulated　by　a　polymer.　In　addition,　the　imaging　effect　was　more　vivid　and　there　was　almost　no　background　interference　of　T　1-　and　T　2-weighted　images　in　vivo,　which　indicated　that　the　Fe3O4@DDT-PMAA　MF　exhibited　excellent　potential　in　MRI　applications.　©　2021　Hongcai　Li　et　al.,　published　by　De　Gruyter　2021.