Abstract: Holographic polymer dispersed ionic liquid (HPDIL) with transmission grating structures was fabricated through holography, utilizing the homogeneous mixture composed of a photoinitibitor, monomers and the ionic liquid (IL) named 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The effects of IL mass fraction on the system viscosity, photopolymerization kinetics, degree of phase separation, optical properties and ionic conductivity of HPDIL were investigated systematically. With an increase of the IL mass fraction from 0.2 to 0.6, the mixture viscosity increased from 6.9 mPa·s to 25.3 mPa·s. Upon exposure to 442 nm monochromic light with an intensity of 20 mW/cm² for 1 200 s, the overall double-bond conversion of monomers decreased from 39% to 18% when increasing the IL mass fraction from 0.2 to 0.6, primarily because of the increased viscosity. The diffraction efficiency of HPDIL increased from 0 to 70.5% when increasing the IL mass fraction from 0.2 to 0.6, and then decreased to 55.0% when further increasing the IL mass fraction to 0.7. The maximum refractive index modulation was 12.3×10^-3 with 0.6 of IL mass fraction. The maximum degree of phase separation reached 38.2% when the mass fraction of IL was 0.5. The HPDIL was able to be applied as solid electrolyte with anisotropic ionic conductivity. The ionic conductivity along the direction parallel to the gratings of HPDIL could be 2.7 times as much as that along the perpendicular direction when the mass fraction of IL was 0.6.