Resistance measurement, P–E hysteresis measurement, and transmission electron microscope and energy dispersive analysis of x-rays (TEM-EDAX) analysis were used to study the resistance failure of lead magnesium niobate-based multilayer ceramic capacitors (MLCC) under dc voltage. It was found that the failure rate of MLCC with 1/9 Pd/Ag internal electrodes was 10 times that of MLCC with 3/7 Pd/Ag electrodes after the temperature–humidity–bias test (THB). Voltage shifts of hysteresis loops showed that an internal bias field between electrodes of MLCC was formed after THB test. Ag diffusion from electrodes into the ceramics during cofiring was examined through TEM-EDAX analysis. It was also found that the degraded specimens could be partially restored after storing under natural condition. On the basis of these results, the failure mechanism was established that oxygen vacancies induced by Ag diffusion accumulated under the external bias field, which increased the concentration of electronic defects, thereby resulting in the resistance failure of MLCC.