In our previous works, it had been found the apparent dielectric constant ∈c′ of α-LiIO3 single crystal increases as a bias DC field is applied in the c direction. And it had been attributed to result of ionic conduction. In this paper, the dielectric properties of KLiSO4 and NaCl single crystals under the action of a DC field have been investigated. Similar to the situation of α-LiIO3 single crystal, the phenomenon of change of the apparent dielectric constant was observed in these crystals. It verified the conclusion mentioned above. We also studied the dependences of this property of ionic conductors on temperatures, frequencies and ambient atmosphere. The condition under which this phenomenon appeares was discussed.