The effect of the amount of the sintering additives, i.e., yttria and alumina, on the microstructure and the high-temperature flexural strengths of silicon nitride with and without the aligned whisker seeds was examined. As the amount of the sintering additives was increased, both width and length of the grains were increased. The high-temperature strengths of silicon nitride with and without the aligned seeds were increased and decreased, respectively, according to the increase of the amount of the sintering additives. When both 4.8 wt% yttria and 2.2 wt% alumina were added to silicon nitride with the aligned whisker seeds, the experimental results indicated that it had three times the flexural strength as silicon nitride without the seeds at 1673 K. During the high-temperature flexure testing, observations did not indicate yielding present in the samples with the aligned whisker seeds, while it was present in samples without the whisker seeds. The reinforcing grains growing from the whisker seeds aligned perpendicular to the crack propagation direction acted as effective obstacles to the crack propagation and improved the fracture toughness.