ZnO varistors are nonlinear resistors used as surge arresters in power transmission and distribution for the protection of electronic devices. Electrical characteristics of these materials have been extensively studied, but their mechanical behavior is not completely understood. It has been suggested that the breakdown of ZnO varistors is related to microstructure heterogeneities and processing defects. These defects are the same that usually control the mechanical strength of ceramic materials. In this work, mechanical properties (flexural strength, fracture toughness, elastic constants, and hardness) of five commercial blocks of ZnO varistors (class I) from different producers were measured and correlated to their microstructure. Pore fraction and size significantly affected the flexural strength.