Abstract Segregation of solute atoms at grain boundaries has been found in many materials and could have a great impact on their properties. However, such important phenomenon has not been observed in early transition metal borides. In the present work, tungsten segregation at grain boundaries was found in ZrB2-SiC-WC ceramics (ZSW), particularly at ZrB2/ZrB2 rather than ZrB2/ZrC grain boundaries. Besides, densely packed dislocations, mainly in a mixed type, were found in many ZrB2 grains. The formation mechanism for W segregation and high-density dislocations in ZSW are discussed, both of which are considered to contribute the excellent high-temperature mechanical properties of ZSW.