Abstract: The silicon dioxide/low density polyethylene (SiO₂/LDPE) nanocomposites were developed by filling SiO₂ nanoparticles into LDPE matrix with melt-blending preparing method to investigate nano-modification method and correlated mechanism for insulation performance improvement of LDPE. The micro-morphology and dispersivity of SiO₂ nanofillers composite in LDPE matrix were characterized by scanning electron microscopy (SEM). The differential scanning calorimetry (DSC) and polarizing microscope (PLM) were employed to testify the effects of SiO₂ nanofillers on the crystallinity and crystal morphology of LDPE matrix. The trap energy level and density were tested by thermal stimulated current (TSC), combining the Weibull statistics of electric breakdown strength to investigate insulation breakdown performance and attributed mechanism. The nano-SiO₂ filling rate can change the crystallinity of composites, while the change will increase the intrinsic structural defects and trap density of LDPE matrix. At the same time, filling nano-SiO₂ particles can introduce deeper trap levels than the intrinsic traps of LDPE matrix. Therefore, the most efficient inhibition to electric breakdown process by trapping charge carriers with largest energy, and thus the maximum breakdown strength are obtained for 3wt% filling rate of SiO₂/LDPE nanocomposites. In comparison with 60 nm SiO₂ nanoparticles, the 30 nm SiO₂ nanoparticles with larger specific surface area filled into LDPE render higher dielectric permittivity from SiO₂-LDPE interface polarization, indicating the more efficient enhancement for electric breakdown strength by higher-density deep traps at larger-area nano-interfaces. Based on dielectric double-layer theory, the reliable models of electron capture and interface structure were put forward so as to reasonably represent the microscopic trap characteristics and macroscopic electric breakdown mechanism of SiO₂/LDPE nanocomposites.