CaCO nanoparticles (nano-CaCO ) can neutralize the acidic pHe of solid tumors, but the lack of intrinsic imaging signal precludes noninvasive monitoring of pH-perturbation in tumor microenvironment. We aim to develop a theranostic version of nano-CaCO to noninvasively monitor pH modulation and subsequent tumor response. We synthesized ferromagnetic core coated with CaCO (magnetite CaCO ). Magnetic resonance imaging (MRI) was used to determine the biodistribution and pH modulation using murine fibrosarcoma and breast cancer models. Magnetite CaCO -MRI imaging showed that nano-CaCO rapidly raised tumor pHe, followed by excessive tumor-associated acid production after its clearance. Continuous nano-CaCO infusion could inhibit metastasis. Nano-CaCO exposure induces tumor metabolic reprogramming that could account for the failure of previous intermittent pH-modulation strategies to achieve sustainable therapeutic effect.