The local detection of magnetic domains of isolated 10 nm Fe3O4 magnetic nanoparticles (MNPs) has been achieved by field-variable magnetic force microscopy (MFM) with high spatial resolution. The domain configuration of an individual MNP shows a typical dipolar response. The magnetization reversal of MNP domains is goveed by a coherent rotation mechanism, which is consistent with the theoretical results given by micromagnetic calculations. Present results suggest that the field-variable MFM has great potential in providing nanoscale magnetic information on magnetic nanostructures, such as nanoparticles, nanodots, skyrmions, and vortices, with high spatial resolution. This is crucial for the development and application of magnetic nanostructures and devices.