Polypropylene (PP) is widely used as capacitor films due to its better dielectric, mechanical, and thermal performance. In order to reduce the cost and size of capacitor, high energy density for PP dielectric is pursued. Since energy density is in quadratic proportion to direct current (dc) breakdown strength for linear dielectric, the enhancement of dc breakdown strength for PP dielectric is a primary choice to improve the energy density. Considering that the incorporation of nano-Al2O3 is an effective method to improve the dc breakdown strength for polymer, it is required to study the dc breakdown strength of PP/Al2O3 nanodielectric. In order to explore the breakdown mechanism, PP/Al2O3 nanodielectrics with different nano-particle contents are prepared by melt blending, and the samples are prepared by hot pressing. Their microstructures are observed by scanning electron microscopic. Isothermal surface potential decay, bulk resistivity, and dc breakdown strength of the samples are also measured. The experimental results show that the energy and density of deep traps, bulk resistivity, and dc breakdown strength first increase and then decrease with the increase in nano-Al2O3 content. The maximum values are obtained at a filer content value of 0.5 wt%, where dc breakdown strength can be increased by about 27%. Based on interface model, the relation between microstructure and trap is investigated. In view of space charge breakdown theory, the mechanism of dc breakdown for PP/Al2O3 nanodielectric is explored by trap parameters. It is indicated that the interface can provide more deep traps in PP/Al2O3 nanodielectric, while the decrease in the energy and density of deep traps can be attributed to the overlap of interfaces in electrical double layer. The increase in the energy and density of deep traps makes more carriers trapped near the injecting contact, thus reducing the effective field for carrier injection due to the internal field generated by the trapped carriers. The reduction of carrier injection can moderate the distortion of field in PP dielectric, consequently, resulting in enhancing the dc breakdown strength.