The structure defects were observed by HREM in α-Si3N4, which was made by hot-pressing method with MgO and LiF as the additives. The observations indicate that the phenomenon of phase seperation occured in the grain boundaries of three grains junction, showing inhomogeneous chemical composition in glassy phase of grain boundaries. In the grains, the stress areas caused by lattice distortion and displacement of (100) planes were found. The distortions of the lattices are so serious in some regions that the hexagonal symmetry in the unit-cell gets lost. The distortion of the lattices could be measured by the deviation of the spacings of (100) planes from the normal value, 6.771?. With HREM we have found electron radiation damage of α-Si3N4 grains at lattice level caused probably by the distortion of the unit-cells of α-Si3N4. As compared with β-Si3N4, α-Si3N4 is less stable thermodynamically. According to our observations of both α-Si3N4 and β-Si3N4 made by hot-pressing method, the defects of the structure in α-Si3N4 are much more than that in β-Si3N4. This indicates that the phenomenon probably arises from easier distortion of the α-Si3N4 lattices than that in β-Si3N4.