The beta-Si3N4 particles were prepared by heating original alpha-Si3N4 powder with rare earth oxide Nd2O3 or Yb2O3 additives at 1600-1700 degreesC for 1.5 h. The transformation ratio Of alpha-Si3N4 was also investigated by XRD. The results showed that Yb2O3 could accelerate the transformation Of Si3N4 more effectively than Nd2O3 and the powder heated at 1700 degreesC with over 4 wt.% Yb2O3 has a high transformation ratio of over 98%. The morphologies of the heated powders were observed by scanning electron microscopy. The results showed that the powder heated at 1700 degreesC with 4 wt.% Yb2O3 had ideal beta-Si3N4 rod-like morphology particles. This heated powder was used as a seed by adding it to the original alpha-Si3N4 powder to prepare self-reinforced Si3N4 ceramic by hot-pressed sintering. The fracture toughness of the seeded Si3N4 ceramics increased to 9.1 MPa m(1/2) from 7.6 MPa m(1/2) of the unseeded Si3N4 ceramics, while the high value of strength was still kept at 1200 degreesC. (C) 2003 Elsevier Science Ltd. All rights reserved.