Improvement of dielectric strength of polymeric insulating materials has been focused on for a long time. We investigated the effects of surface morphology and crystalline structure of LDPE on its breakdown field strength. Polyester film（PET） and mica（mica） sheets were used as a compression mold preparation of LDPE slice samples. Atomic force microscopy（AFM） tests show that mica with atomic level flatness can improve the surface morphology of LDPE samples and form regular and larger spherulites. The results of differential scanning calorimetry（DSC） test show that the crystallinity of mica prepared sample is about 24% higher than that of PET’s

and the melting temperature of slow cooling sample is about 3 ℃ higher than that of rapid cooling. The results of dynamic mechanical analysis（DMA） test show that mica can improve the crystallization of polyethylene

and the peak value of relaxation loss is decreased by the increase of molecular rigidity. The mechanical relaxation peak will move to about 85 ℃ at high temperature

which promotes the electrical strength in high temperature. The results of breakdown test show that the AC and DC breakdown strength of LDPE samples prepared by mica is higher than that by PET

and the AC breakdown strength of LDPE samples slow cooled by mica is 17% higher at 90 ℃. The surface roughness of the mica surface is lower than that of PET. The simulation of the electric field by COMSOL shows that the electric field is less enhanced by the surface roughness of mica prepared LDPE samples.