In this paper in situ toughened Yb alpha-SiAlON ceramics with elongated grains were prepared by two-step hot-pressing. The phase transformation and microstructure development of the samples during the two-step sintering were discussed in detail. It was found that at low temperatures, the formation of intermediate phases reduced the amount of liquid phase and thus slowed down the dissolution and mass transportation processes. In this way, the grain growth was depressed and fine grains were obtained. At the elevated temperature, the intermediate phases would be dissolved, resulting in abundant liquid phase and fast anisotropic grain growth. By the two-step sintering procedure, the nucleation and grain growth could be well manipulated and coarse elongated grains could be produced. These elongated grains improved the fracture toughness of the sintered ceramics via the mechanisms such as grain pull-out and debonding, which were clearly observed by SEM. In addition, the preferred orientation of the elongated a-SiAlON grains was also studied in the light of their growth habit. (c) 2005 Elsevier B.V. All rights reserved.