Compared with other medical imaging techniques, magnetic resonance imaging (MRI) has an obvious disadvantage of low sensitivity, in other words, the injection dosage of the probe (contrast agent) is very high. In this paper, we developed c(RGDyK)-conjugated KMnF₃ nanocubes as a tumor targeting T₁ contrast agent. The short cyclic peptide (c(RGDyK)) was conjugated with carboxylic terminated KMnF₃ nanoparticles via 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) coupling. The nanoparticles have an appropriate hydrodynamic size and narrow size distribution. Furthermore, they showed high in vitro bio-stability and biocompatibility. Application of the c(RGDyK)-conjugated nanoparticles for in vivo MR imaging of tumors profoundly enhanced the image contrast to almost double. More importantly, the injection dosage required is one to two orders lower than that of a commercial or reported contrast agent. The c(RGDyK) short peptide was proven to play a targeting role, and c(RGDyK)-conjugated KMnF₃ nanoparticles have a potential application for the diagnosis of tumors.