In this work, multi-walled carbon nanotubes (MWNTs)/epoxy composites were prepared using an electron beam curing technique and three kinds of MWNTs with different aspect ratio was used as fillers.The dielectric properties of these MWNTs /epoxy composites were studied.At low frequencies, the dielectric permittivity of the composites with 0.1 wt% long MWNTs increases rapidly and the value of the dielectric permittivity at 100Hz reaches 58.4, which is 11 times the corresponding value of the pure epoxy.The length and diameter of MWNTs had a clear impact on the dielectric properties of MWNTs/epoxy composites.