A technique that allows direct optical imaging of nanostructures and determines quantitatively geometric nanofeatures beyond the classical diffraction limit by using high-refractive index glass microspheres is introduced. The glass microsphere is put on the nanostructure that is immersed in oil. When illuminated by conventional oil-immersion microscope objective, a magnified virtual image of the sample is projected by the microsphere and recorded by the same objective. The image reveals the sub-diffraction nanofeatures of the sample due to a highly focused focal spot generated by the microsphere, i.e. a so-called "photonic nanojet". Experimental results on nanostructures demonstrates a resolution of ~ λ/4 - λ/7, where λ is the illumination wavelength, by using a 60 μm glass microsphere and a normal wideband halogen lamp as illumination source.