Accurate assessment of the tumor microenvironment (TME) reflects the dynamic progress of tumor diseases and help establish a more comprehensive diagnosis and efficacy evaluation. However, monitoring the TME mainly relies on interstitial fluid pressure (IFP) and pathological detections, which are invasive detection methods and could introduce errors. Losartan, an angiotensin Ⅱ receptor blocker, can reduce the collagen fiber production and enhance drug delivery, especially in nanomedicine, by improving interstitial transport in tumors. The purpose of this study was to determine whether magnetic resonance imaging (MRI) of magnetic iron oxide nanoparticles (MNPs) could detect changes in the TME and drug uptake in 4T1 mice treated with losartan.