An organic/inorganic composite separator includes a porous substrate having a plurality of pores; and a porous coating layer formed on at least one surface of the porous substrate with a plurality of inorganic particles and a binder polymer. The binder polymer is a copolymer including: (a) a first monomer unit having a contact angle to a water drop in the range from 0.degree. to 49.degree.; and (b) a second monomer unit having a contact angle to a water drop in the range from 50.degree. to 130.degree.. This organic/inorganic composite separator has excellent thermal stability, so it may restrain an electric short circuit between a cathode and an anode. In addition, the separator may prevent inorganic particles in the porous coating layer from being extracted during an assembling process of an electrochemical device, thereby improving stability of an electrochemical device.