A separator that includes a porous polymer substrate, a first porous coating layer and a second porous coating layer. First inorganic particles contained in the first porous coating layer have a lower hardness as compared to the second inorganic particles contained in the second porous coating layer. Since the separator is provided with at least two porous coating layers including inorganic particles having a different hardnesses, it is possible to increase the dielectric breakdown voltage, and thus to provide a battery with improved safety. It is also possible to reduce deformation of the separator caused by heat or pressure.