Polycarbonate (PC) composites reinforced with potassium titanate (K2Ti6O18) whiskers were blended in a twin-screw extruder followed by injection molding. The surface of whiskers was treated with tetrabutyl orthotitanate prior to blending. The effects of potassium titanate whisker additions on the tensile, impact, and thermal properties of PC were investigated. Tensile tests showed that the stiffness of composites markedly improved with increasing whisker content. However, potassium titanate whiskers were ineffective to reinforce PC because these whiskers promoted chemical decomposition of PC matrix during compounding. Consequently, the torque values of PC/K2Ti6O13 composites were much lower than that of PC. Moreover, torque measurements revealed that titanate coupling agent also facilitated decomposition of PC during blending. The mechanisms responsible for the degradation of PC matrix of the surface-treated PC/K2Ti6O13 composites are discussed.