The objective of the proposed research is to understand the degradation of varistors used for dc circuit breaker (DCCB) applications. The uniqueness about varistors in DCCB applications includes zero stress at rest, frequent impulses stress from DCCB operation, and system determined impulse waveform. To study the influential material properties with respect to degradation, failure, and lifetime of DCCB varistors, a first principle model and a 2D network model have been developed. In addition, to learn DCCB varistor degradation mechanism, failure modes, and lifetime expectation, a 1.2kV-rated accelerated DCCB varistor degradation testing platform was designed and built. Four groups of varistor samples have been tested under different peak current and conduction time conditions to get preliminary results, which show different lifetime values from the specification sheet data. Therefore, the work to study degradation of varistors used for dc circuit breaker applications is proposed, and future work includes building full scale (4.5kV) tester, developing full scale 3D discreate and/or FEA model, and performing comprehensive material characterizations and postmortem studies to learn the degradation mechanism.