Abstract: This paper focused on the effect of mechanical strains on space charge and breakdown characteristics of polypropylene (PP) insulation for high-voltage cables. PP insulation samples were prepared by isothermal crystallization treatment whose DC conductivity, space charge, and breakdown characteristics were investigated under mechanical strains. The mechanism of mechanical strains on the electrical properties of PP insulation was analyzed by characterizing the microscopic crystalline morphology. The results show that mechanical strains of PP insulation lead to a higher conductivity, an increased space charge density, and a decrease of DC breakdown strength. The isotactic-PP (iPP)/ syndiotactic-PP (sPP) mixture, with a higher elongation at break, is more outstanding on DC conductivity and breakdown strength than that of iPP insulation at the same strain rate. The isothermal crystallization treatment affects the crystallinity and crystalline morphology of PP insulation, which not only improves the impact resistance, but also elevates DC breakdown strength. In conclusion, the mechanical strains accelerate the injection of space charge and decrease breakdown strength, resulting from defects at the crystal- amorphous interface, following with deformation and rupture of spherocrystals in PP insulation.