A great many boron nitride (BN) nanoparticles were synthesized from boron oxide by high-energy ball milling and annealing process at 1200°C. The sample was characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, selected area electron diffraction, high-resolution transmission electron microscopy and the X-Ray energy dispersive spectroscopy. The results showed that the morphology of most synthesized BN nanoparticles is elliptical plate-like, while the others are circular plate-like. The BN nanoparticles are uniform at the diameter from 20nm to 100nm, and most of them are around 70nm in diameter. The thickness of them is about 10nm estimated from SEM image.