An organic/inorganic composite separator, comprising, (a) a polyethylene porous substrate having a porosity of 45% and a thickness of 16 μm; and (b) a porous active layer containing a mixture of Al 2 O 3 particles having a diameter of 400 nm and a density of 4.123 g/cm 3 and a binder polymer being a mixture, in identical parts by weight, of polyvinylidenefluoride-co-hexafluoropropylene having a contact angle to a water drop of 100° and cyanoethylpolyvinylalcohol having a contact angle to a water drop of 30°, wherein a weight ratio of the binder polymer/Al 2 O 3 is 10/90, with which the polyethylene porous substrate is coated, wherein the porous active layer has a peeling force of 27.44 N/m (28 gf/cm), and a thermal shrinkage of the separator after being left alone at 150°C for 1 hour is 20% in a machine direction or in a transverse direction, wherein the coating thickness of the porous active layer is 4 μm on one surface of the polyethylene porous substrate, wherein a pore size in the porous active layer is 0.5 μm and a porosity is 58%, the weight of the porous active layer is 16 g/m 2 per unit area, and the number of Al 2 O 3 particles in the porous active layer is 5 x 10 19 /m 2, wherein, after a sample film was made using a corresponding binder polymer, a distilled water drop is fallen thereon, the contact angle to a water drop is measured using a contact angle measurer model CA-DT-A under the condition of 23°C and 50% RH, wherein the contact angles are measured at left and right points of each of three sample films, and six measured values are averaged and set as a contact angle,, wherein the distilled water drop has a diameter of 2 mm, and the contact angle value displayed on the measurer shows a contact angle measured 1 minute after the distilled water drop is fallen, wherein the peeling force is measured by firmly attaching the organic/inorganic composite separator to a glass plate using a transparent double-sided tape, the separator having a width of 1.5 cm and a length of 6 to 8 cm, and measuring a force required for separating the porous active layer from the polyethylene porous substrate using a tensile force measuring device. (a) a polyethylene porous substrate having a porosity of 45% and a thickness of 16 μm; and (b) a porous active layer containing a mixture of Al 2 O 3 particles having a diameter of 400 nm and a density of 4.123 g/cm 3 and a binder polymer being a mixture, in identical parts by weight, of polyvinylidenefluoride-co-hexafluoropropylene having a contact angle to a water drop of 100° and cyanoethylpolyvinylalcohol having a contact angle to a water drop of 30°, wherein a weight ratio of the binder polymer/Al 2 O 3 is 10/90, with which the polyethylene porous substrate is coated, wherein the porous active layer has a peeling force of 27.44 N/m (28 gf/cm), and a thermal shrinkage of the separator after being left alone at 150°C for 1 hour is 20% in a machine direction or in a transverse direction, wherein the coating thickness of the porous active layer is 4 μm on one surface of the polyethylene porous substrate, wherein a pore size in the porous active layer is 0.5 μm and a porosity is 58%, the weight of the porous active layer is 16 g/m 2 per unit area, and the number of Al 2 O 3 particles in the porous active layer is 5 x 10 19 /m 2,