Label-free single-nanoparticle detection is crucial for the fast detection of nanoparticles and viruses in environmental monitoring and biological sciences. In this Letter, benefiting from the leakage radiation that transforms the near-field surface plasmon polariton (SPP) distribution along the interface to the far field, we demonstrated the plasmonic imaging of single polystyrene nanoparticles with a particle size down to 39 nm. The imaging is composed of the localized enhancement and interference of SPPs. The localized enhancement is the result of the accumulation of charges around the nanoparticle, and it is connected to the size and refractive index of nanoparticles. The interference is induced by the coupling between the incident SPPs and the scattered SPPs, verified by extracting the interference fringe periodicity to be half of the SPP wavelength. Our study provides an in-depth physical understanding of plasmonic imaging of single nanoparticles, which paves the way for a fast identification of nanomaterials.