New lithium ion composite electrolyte, LiI-Li2S-La2O2Sm (m = 1, 2) was synthesized from the binary Li2S-LaOI system through solid state reaction. The lithium ion conductive property was investigated by AC impedance spectroscopy. And the highest conductivity of the obtained electrolyte at room temperature was found to be 3.0 x 10(-6) S cm(-1). The notable ionic conduction was attributed to the in situ formed amorphous LiI. (C) 2008 Elsevier B.V. All rights reserved.