In this work, 800 ppm of Zr4+ dopants were added to Al2O3-5 vol% SiC particle composite. Zr4+ doping led to a weak Al2O3 grain-boundary bonding so that the fracture mode changed from transgranular in undoped composite to intergranular in Zr4+-doped composite. The fracture mode change increased the fracture toughness of the composite. Transmission electron microscopy and energy-dispersive spectroscopy examinations revealed that the weak grain-boundary bonding in the doped composite was caused by the segregation of Zr4+ and Si4+ ions at the Al2O3 grain boundary.