High temperature flexural strength of ZrB220 vol% SiC ceramics (ZS) up to 1600 degrees C in high purity argon atmosphere was significantly improved by adding 5 vol% WC, but degraded when 5 vol% ZrC was added. ZrB220SiC5WC ceramic (ZSW) has a very high strength (mean +/- SD) of 675 +/- 33 MPa at 1600 degrees C, and also an elastic and transgranular fracture mode was observed. According to the analysis of the fracture modes and crack origins in ZSW ceramics, the improvement in strength above 1000 degrees C was attributed to the removal of the oxide impurities from grain boundaries.