A separator for an electrochemical device. The separator includes a porous coating layer formed from an acid, which is crosslinkable. As a result, the separator has increased heat resistance, safety and physical strength, shows improved peel strength between the porous substrate and the porous coating layer, and prevents separation of the inorganic particles from the porous coating layer. In addition, the separator shows an improved crosslinking degree so that the added amount of a crosslinkable binder resin may be reduced. Thus, it is possible to increase the added amount of a non-crosslinkable resin, inorganic particles, or both. Even when using a small amount of a crosslinkable binder resin, it is possible to provide both an effect of improving heat resistance and an effect of improving adhesion.