The separator is a porous membrane located between the negative and the positive electrodes. In this work, a nano-fiber reinforced composite separator was developed. Compared with the commercial polyolefin separator, the composite separator showed superior (a) dimensional stability at elevated temperatures relative to conventional separators and (b) wettability by the liquid electrolyte. After being saturated with a commercial LiPF6-ethylene carbonate-dimethyl carbonate electrolyte, the composite separator enabled a high effective ionic conductivity ( σ e f f ) of 1.25 mS/cm. A stable cycle performance and an improved rate capability have been observed in the coin cells with the composite separator. This initial study shows that this type of composite membranes can be a promising alternative separator for lithium ion batteries.