The detection and diagnosis of early hypoxic tumor is the key for successful cancer therapy, which remains a challenge for radiologists because contrast agents could hardly reach it. Herein, a tumor-targeting MRI contrast agent was prepared by conjugating the small-molecule inhibitor of carbonic anhydrase (CA IX) that is overexpressed in hypoxic tumor, mafenide, onto the surface of Fe3O4 magnetic nanoparticles (MNPs) to improve the sensitivity of MRI for early tumor diagnosis. The tumor-targeting MNP (Fe3O4-SO2NH2) showed enhanced MRI contrasting performance both in vitro and in vivo in comparison with the non-targeting control, which made it a promising contrast agent for the detection and diagnosis of early hypoxic tumor.