β-Si₃N₄ powders containing 0.5mol% Y₂O₃ and 0.5mol% Nd₂O₃ were gas-pressure sintered at 2000°C for 2, 4, and 8h. The obtained materials presented the microstructure of self-reinforced composites consisting of some large rod-like grains in a fine matrix, as it was observed by scanning electron microscopy of plasma-etched surfaces. Two-dimensional distributions of grain diameter, grain length, and cross-sectional area were quantitatively analyzed using an image analyzer. The distribution curve of grain number and grain cross-sectional area did not reflect the presence of large grains, and therefore this method is not suitable for evaluation of self-reinforced materials. The volume fractions of large grains and fine matrix grains were estimated from the relation between the grain size and the two-dimentional distribution curve of grain cross-sectional area.