Provided is a separator for an electrochemical device. The separator includes a porous substrate made of a porous polymer film having an excellent compressibility and permanent strain, wherein the porous substrate has excellent physical strength and durability and ensures a high breakdown voltage while using a heat resistant layer having a small thickness, and thus shows a low possibility of short-circuit generation. In addition, the separator may further include a heat resistant layer including inorganic particles, on the surface of the porous substrate. Herein, it is possible to further improve the compressibility, maximum compressibility and permanent strain characteristics depending on types of inorganic particles.