<p>A novel grain boundary character was identified in a silicon nitride ceramic ball (22.5 mm in diameter) fabricated via hot isostatic pressing (HIP) at 1780 °C and 100 MPa. Before HIP, the raw <i>α</i>-Si₃N₄ powder, Y₂O₃ + Al₂O₃ sintering aids and small amount of TiN powder were mixed by ball-milling for 24 hours and then granulated by dry pressing and cold isostatic pressing. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron dispersive spectrum (EDS) observations and analyses indicate that the HIP-ed Si₃N₄ ball has a very fine microstructure with an averaged grain size around 380nm, and the grain boundaries were characterized. Based on the compositional contrast under TEM and the content of the glass phase, the grain boundaries were classified into three types. The first type has a significant accumulation of amorphous glass phase. The second type has trace amounts of glass phase in the form of intergranular glass film. The third type contains no residual amorphous glass phase and is a pure silicon nitride grain boundary; this is the new finding in the present work. It suggests that the third-type grain boundary can effectively interrupt the connectivity of the "fragile" glass phase network created by the first- and second-type grain boundaries; therefore, it could positively affect the mechanical properties of the silicon nitride ceramic ball.</p>