Low frequency dielectric measurements are performed on a nonchiral liquid crystal(8CB) for two different surface treatment of rubbing and no rubbing for 4.5 ㎛ thickness as a function of temperature. Relaxation peak frequencies are observed at ~10 Hz for both samples. When the external field amplitude was increased the relaxation peak frequency was shifted to lower frequency. To explain this odd result of experimental observations we performed a numerical simulation, where the ionic impurities were assumed to encounter ionization-recombination processes in addition to a translational diffusion. We could confirm two separate contributions from ionic impurities to the low frequency dielectric relaxations in this system, that is, from fast ions in a single particle diffusion and slow ions in the ionization-recombination assisted diffusion. We also calculated dielectric constant in the limit of 0 mobility for slow ion and from this calculation we can obtain diffusion constant of impurity.