A Gd-doped ultrasmall CeO 2 contrast agent was prepared with high longitudinal relaxivity ( r 1 = 10.1 mM -1 s -1 , 7.0 T) through rationally regulating the crystallinity and surface coatings, providing a new paradigm for optimizing MRI performance. Moreover, responsive photoacoustic imaging (PAI) was established via tumor microenvironment-triggered oxidation, affording dual-modal imaging.