A proper powder preparation was used to develop alpha-Sialon single crystals as seeds. The microstructure and fracture toughness of seed-containing alpha-Sialon ceramics sintered by hot-pressing were investigated. The specimen without seeding consisted of fine grains and a small amount of coarse grains. Specimens seeded with alpha-Sialon single crystal particles presented a large amount of elongated alpha-Sialon grains. The aspect ratio and the amount of elongated alpha-Sialon grains can be tailored by using different sizes and amounts of the seeds. The fracture toughness of seed-containing alpha-Sialon ceramics is improved, which is attributed to grain pullout and bridging of elongated grains.