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In dielectric science, the electrical breakdown strength of a polymeric material significantly decreases with elevated temperatures, which restricts the development of advanced electrical and electronic applications toward miniaturization. In the present study, to clarify the temperature-dependent DC breakdown mechanisms of epoxy resin (EP)/TiO2 nanocomposites, the effects of nanoparticle incorporation and temperature on charge transport and molecular chain dynamics were studied. The results indicate that space charge accumulation and electric field distortion are reduced by nanoparticle incorporation to enhance the deep trap level, while space charge accumulation, electric field distortion, and molecular displacement are all accelerated as temperature increases. To further investigate the influence of carrier traps and molecular chain dynamics on temperature-dependent breakdown, a DC breakdown simulation model that involves bipolar charge transport, molecular chain dynamics, and breakdown criterion equations is established. The calculated breakdown strengths of EP/TiO2 nanocomposites show great accordance with the experimental results, which indicates that temperature-dependent DC breakdown mechanisms are dominated by the synergetic effects of carrier traps and segment chain dynamics. Through the analysis of the breakdown model, a transition of the dominant mechanism (from segment chain to backbone dynamics) near the glass-transition temperature for DC breakdown of EP/TiO2 nanocomposites is discovered.