The morphology and electrical breakdown of polypropylene (PP) are investigated. Experimental results show that electrical strength depends on the size of microvoids and their distribution in PP instead of the macrostructure parameters such as crystallinity and density, and the microvoids with diameters of 100-1000 AA affect electrical strength remarkably. On the basis of the morphology and electronic structure of PP, two necessary conditions for electrical breakdown are proposed. The energy required for electrons to break chemical bonds in electrical breakdown is the band gap of the polymer instead of the bond energy. >