Heat treatment effect of polyethylene (PE) separators was investigated after storage at 80, 100 and $120^{circ}C$ for 1 h. All the samples showed enhanced tensile strength and modulus after heat treatment, but thermal shrinkage up to 15% was observed in PE films having newly formed dimple structure on the surface of fiber after annealed at 100 and $120^{circ}C$. Although there was 5% of thermal shrinkage after annealing at $80^{circ}C$, no such serious changes in PE fiber was observed. Furthermore, the separator was found to have enhanced cell performance with 1.3 and 2.3 times higher tensile strength and modulus after heat treatment at $80^{circ}C$ for 1 h.