Abstract: Nano-ZnO and nano-montmorillonite (MMT) particles were added to improve the dielectric performance of low density polyethylene (LDPE). And the coupling agent was used in surface modification of nanoparticles. The composites of nano-ZnO/LDPE and nano-MMT/LDPE were prepared by melt blending method. The sample was characterized by XRD, FTIR and DSC. The alternating current breakdown properties of composites were investigated. The space charge tests were performed for the sample. Results show that the interface binding between nano-particles modified by coupling agent and polymer is improved, and the dispersion of nano-particles in matrix is more uniform, the crystallization rate of composites increases and crystalline structure becomes more perfect. Adding nanoparticles can increase the breakdown field strength of LDPE in different degrees. When the mass fraction of nano-ZnO and nano-MMT is 3wt%, respectively, the breakdown field strength of composites appear the highest values, which are 11.0% and 10.3% higher than those of origin LDPE, respectively. Both nano-ZnO and nano-MMT play an important role in inhibiting space charge, and the effect of nano-ZnO seems more obvious.