In this work, a T 2 -T 1 switchable superparamagnetic iron oxide nanoprobe with a pH/H 2 O 2 dual response was obtained using a microemulsion method. This novel method for the controllable assembly of small iron clusters followed by their independent modification was reported, which could not be prepared by common synthetic methods. The size of the assembled nanoprobe was uniform and controllable, with a stable T 2 magnetic resonance imaging (MRI) signal under a single condition. When the nanoprobe was exposed to the tumor environment, the higher H + and H 2 O 2 concentrations at the tumor site could dissociate the nanoprobe and redisperse into small iron clusters. When this occurred, the T 2 MRI signal was converted into a T 1 MRI signal, achieving specific detection of tumors by a pH/H 2 O 2 dual-response T 2 -T 1 MRI.