ZrB2-ZrN1-x composites were in situ synthesized from Zr and BN powders by hot-pressing at high temperatures. Thermodynamic calculation indicates that ZrN will be formed preferentially than ZrB2 in Zr-BN system. Three samples with Zr/BN molar ratios of 3:2, 3.5:2 and 2:1 were investigated at temperatures above 1650 degrees C. All mixtures of Zr and BN transformed to ZrB2-ZrN1-x composites completely without any other detectable phases. Nonstoichiometric zirconium nitride, ZrN1-x, is supposed to be formed in 3.5:2- and 2:1-samples. The microstructural morphology of well-sintered ZrB2-ZrN1-x composites is characterized by quadrate column-shaped ZrB2 distributed evenly in the interwoven acicular ZrN1-x matrix. A certain amount of hollow rectangular-shaped ZrB2 with open ends is found in 3.5:2-sample hot-pressed at 1700 degrees C, while some large spherical particles with lots of acicular ZrN1-x, sticked on its surface are observed in 2:1-sample hot-pressed at 1800 degrees C. Excessive Zr compared to the stoichiometric Zr/BN molar ratio of 3:2 will facilitate the densification process. Acicular ZrN1-x, is apparently beneficial to the improvement of bending strength and fracture toughness of ZrB2-ZrN1-x composites. (c) 2006 Springer Science + Business Media, Inc.