Multiwalled carbon nanotubes (MWNT) were modified by imidazolium ionic liquid (IL) to improve the dispersibility of MWNT and the interphase property between MWNT and LDPE. The physical cation-π interaction between the IL and the MWNT was evidenced by Raman spectroscopy. LDPE based composites filled with IL-functionalized MWNT (MIL) and MWNT were fabricated by the melt mixing and hot pressing method. The structure and dielectric properties of the composites were measured and analyzed. Compared with MWNT/LDPE composites, the MIL/LDPE composites exhibit a lower loss tangent and a higher dielectric permittivity. The interphase between MIL and LDPE is in charge of the improvement of dielectric permittivity, and the nonconducting organic segment of the IL supports the low loss tangent in the MIL/LDPE composites. The temperature dependence of complex permittivity is minimal in the MIL/LDPE composites at low temperature.