beta-Si3N4 ceramics sintered with Yb2O3 and ZrO2 were fabricated by gas-pressure sintering at 1950degreesC for 16 h changing the ratio of "fine" and "coarse" high-purity beta-Si3N4 raw powders, and their microstructures were quantitatively evaluated. It was found that the amount of large grains (greater than a few tens of micrometers) could be drastically reduced by mixing a small amount of "coarse" powder with a "fine" one, while maintaining high thermal conductivity (>140 W.(m.K)(-1)). Thus, this work demonstrates that it is possible for beta-Si3N4 ceramics to achieve high thermal conductivity and high strength simultaneously, by optimizing the particle size distribution of raw powder.