The physical effects of a polyethylene (PE) surface on the electrochemical and thermal properties of a PE separator coated with polyvinylidene fluoride (PVDF)–12wt% hexafluoropropylene (HFP) co-polymer have been investigated for use in lithium ion batteries. To change the surface property of the PE separator, it is treated using gamma ray irradiation. In comparison with the non-irradiated separator, the irradiated separator shows strong affinity for polar solvents due to the carbonyl band formed during gamma ray irradiation. And the PVDF–12wt%HFP co-polymer is coated on the both sides of the non-irradiated and irradiated separators using a dip-coating process. It is clearly observed that the ionic conductivity of the irradiated separator coated with PVDF–12wt%HFP is higher than that of the non-irradiated separator coated with the same one. Based on the activation energy calculation, the ionic conductivity is found to strongly depend on the characteristics of the interfaces between them. In addition, rate discharge property and thermal stability of the irradiated separator coated with PVDF–12wt%HFP are much better than those of the non-irradiated one. The improvement of the thermal stability could be attributed to cross-linking of the support PE separator using gamma ray irradiation.