The dielectric properties of liquid crystal materials containing ionic impurities are analyzed in terms of space charge polarization. The theoretical expressions of the dielectric constant and the dielectric loss factor are derived by solving a diffusion equation for mobile ions under an AC field. Numerical calculations are performed using the theoretical expressions and the frequency dependent dielectric behaviors are simulated. The calculated curves for the dielectric constant and the dielectric loss factor exhibit a dielectric dispersion and a dielectric absorption respectively in a low frequency region, and successfully trace the observed curves obtained for liquid crystal materials 4-n-pentyl-4′-cyanobiphenyl (5CB) by means of curve fitting with the parameters of the diffusion coefficient and the density of mobile ions.