A transmission electron microscope study has been made of a silicon nitride component with 6 w/o yttrium oxide as a sintering aid hot isostatically pressed (HIP) with an encapsulation glass of borosilicate. The TEM study concentrated on the interface region between ceramic and glass. Two different types of hexagonal boron nitride were formed near the interface. One, with a textured structure, seemed to nucleate heterogeneously on the surfaces of silicon oxynitride grains. The (001) planes of the crystals extended outwards, giving a thickness of approximately 0.5 microns. The other type formed as hexagonally shaped grains separate from the first type and appeared to have grown as several segments in different directions around a nucleus. In each segment BN layers are parallel to each other and perpendicular to their common [001]BM direction. This second type of BN crystal was also detected a little further from the surface within the silicon nitride. The volume fraction of additive glassy phase tended to be lower in this surface region than in the bulk. Possible mechanisms of prevention of encapsulation glass penetration into the porous ceramic component during HIP were discussed