Disclosed is a separator for a lithium secondary battery comprising a porous substrate and an organic/inorganic composite porous coating layer on the surface, in the pore, or on the surface and in the pore of the substrate, wherein the organic/inorganic composite porous coating layer comprises heat-absorbing inorganic particles, a binder polymer, and inorganic particles having a dielectric constant of 5 or more, which include at least one selected from the group consisting of SrTiO3, SnO2, CeO2, MgO, NiO, ZnO, Y2O3, ZrO2, Al2O3, TiO2, and BaTiO3, wherein the heat-absorbing inorganic particle is at least one particle selected from the group consisting of antimony trioxide (Sb2O3), antimony tetraoxide (Sb2O4), antimony pentaoxide (Sb2O5), aluminum hydroxide (Al(OH)3), magnesium hydroxide (Mg(OH)2), H3BO3, HBO2, ZnSnO4, ZnSnO3, and ZnSn(OH)6. An electrochemical device including the separator is also disclosed. The separator using the heat-absorbing inorganic particles as a component for forming or coating the separator can ensure excellent thermal safety and minimizes degradation of the quality of a battery.