A separator includes a monolayer-type polyolefin-based micro-porous film having a porosity of 40 to 60%, an average pore diameter of 60 nm or less, and an air permeability of 350 s/100 mL or less; and a porous coating layer formed on at least one surface of the micro-porous film and made of a mixture of a plurality of inorganic particles and a binder polymer. An electrochemical device having the above separator has excellent thermal stability and allows a high power while minimizing the occurrence of leak current.