Breakdown of epoxy composites is easy to be triggered as the temperature is elevated. In order to improve the DC breakdown strength of epoxy composites at elevated temperature and explore the DC breakdown mechanism, functional nano-titania (TiO2) particles were incorporated into the epoxy matrix with different filler loadings, molecular chain dynamic characteristics were analyzed by dielectric relaxation spectrum analysis, free volumes of epoxy nanocomposites were evaluated by thermal expansion dilatometer, and DC breakdown strengths of samples were tested at 413 K. Results indicate that DC breakdown strength first increases and then decreases with nanoparticle filler loadings, and a 10.89% improvement of DC breakdown strength is found compared to pristine epoxy resin. The breakdown strength of epoxy resin at elevated temperature is determined by the expansion properties of free volume in the interfacial region between the epoxy matrix and nanoparticles. When incorporating a small amount of nanoparticles, free volume is difficult to expand due to the strong interactions between molecular chains and nanoparticles, the fraction of free volume decreases, and long molecular chains of epoxy are hard to move, and thus DC breakdown strength increases. While further adding nanoparticles, interfacial regions of nanoparticles overlap and free volumes are likely to expand by thermal stimulation in the overlap region, which accelerate molecular chain dynamics and improve free volume fraction, and DC breakdown strength increases. It can be found that DC breakdown strength at an elevated temperature can be enhanced by tailoring free volume through incorporating proper content of functional nanoparticles.