Abstract: The investigation were mainly focused on the phase composition, microstructure and mechanical properties of fully-dense Re-α/β-Sialon (Re=Sm, Nd) ceramics prepared from Powder mixtures of Si₃N₄, AlN, Al₂O₃ and Sm₂O₃ (or Nd₂O₃) by gas pressure sintering (GPS).Its microstructure was observed by SEM,TEM and HREM. The results show that whisker-like β-Sialon grains and equal-axed α-Sialon grains intersected each other as main crystalline phase, with a small amount of glassy phase and crystalline phase on grain boundaries. These elongated β-Sialon grains offer the major contribution to the unique mechanical properties of the present material. It also has been found that the Sm-α-Sialon is more liable to be formed with the stabilizing agent Sm₂O₃.