An organic/inorganic composite porous separator for an electrochemical device, which comprises: (a) a porous substrate having pores; and (b) an organic/inorganic composite porous active layer coated directly on a surface of the substrate, the active layer comprising a mixture of inorganic particles and a binder polymer, wherein the inorganic particles are present in the mixture of inorganic particles with the binder polymer in an amount of 50-99 wt% based on 100 wt% of the mixture, wherein the inorganic particles in the active layer are interconnected among themselves and are fixed by the binder polymer and permit interstitial volumes to be formed among them, and interstitial volumes among the inorganic particles form a pore structure that permits lithium ions to move therethrough, wherein, as the size of the inorganic particles increases, interstitial distance among the inorganic particles increases, thereby increasing the pore size, wherein, as the weight ratio (I/P) of the inorganic particles to the binder polymer increases, porosity of the organic/inorganic composite porous separator increases, wherein the inorganic particles are at least one selected from the group consisting of: (a) inorganic particles having a dielectric constant of 5 or more; and (b) inorganic particles having lithium ion conductivity, wherein the inorganic particles having a dielectric constant of 5 or more are BaTiO 3, Pb(Zr,Ti)O 3, Pb 1-x La x Zr 1-y Ti y O 3 (PLZT), Pb(Mg 3 Nb 2/3)O 3 -PbTiO 3 (PMN-PT), hafnia (HfO 2), SrTiO 3, SnO 2, CeO 2, MgO, NiO, CaO, ZnO, ZrO 2, Y 2 O 3, Al 2 O 3 or TiO 2, and wherein the inorganic particles having lithium ion conductivity are at least one selected from the group consisting of: lithim phosphate (Li 3 PO 4), lithium titanium phosphate (Li x Ti y (PO 4) 3, 0<x<2, 0<y<3), lithium aluminium titanium phosphate (Li x Al y Ti z (PO 4) 3, 0<x<2, 0<y<1, 0<z<3), (LiAlTiP) x O y type glass (0<x<4, 0<y<13), lithium lanthanum titanate (Li x La y TiO 3, 0<x<2, 0<y<3), lithium germanium thiophosphate (Li x Ge y P z S w, 0<x<4, 0<y<1, 0<z<1, 0<w<5), lithium nitrides (Li x N y, 0<x<4, 0<y<2), SiS 2 type glass (Li x Si y S z, 0<x<3, 0<y<2, 0<z<4) and P 2 S 5 type glass (Li x P y S z, 0<x<3, 0<y<3, 0<z<7). (a) a porous substrate having pores; and (b) an organic/inorganic composite porous active layer coated directly on a surface of the substrate, the active layer comprising a mixture of inorganic particles and a binder polymer, wherein the inorganic particles are present in the mixture of inorganic particles with the binder polymer in an amount of 50-99 wt% based on 100 wt% of the mixture, wherein the inorganic particles in the active layer are interconnected among themselves and are fixed by the binder polymer and permit interstitial volumes to be formed among them, and interstitial volumes among the inorganic particles form a pore structure that permits lithium ions to move therethrough, wherein, as the size of the inorganic particles increases, interstitial distance among the inorganic particles increases, thereby increasing the pore size, wherein, as the weight ratio (I/P) of the inorganic particles to the binder polymer increases, porosity of the organic/inorganic composite porous separator increases, wherein the inorganic particles are at least one selected from the group consisting of: (a) inorganic particles having a dielectric constant of 5 or more; and (b) inorganic particles having lithium ion conductivity, wherein the inorganic particles having a dielectric constant of 5 or more are BaTiO 3, Pb(Zr,Ti)O 3, Pb 1-x La x Zr 1-y Ti y O 3 (PLZT), Pb(Mg 3 Nb 2/3)O 3 -PbTiO 3 (PMN-PT), hafnia (HfO 2), SrTiO 3, SnO 2, CeO 2, MgO, NiO, CaO, ZnO, ZrO 2, Y 2 O 3, Al 2 O 3 or TiO 2, and wherein the inorganic particles having lithium ion conductivity are at least one selected from the group consisting of: lithim phosphate (Li 3 PO 4), lithium titanium phosphate (Li x Ti y (PO 4) 3, 0<x<2, 0<y<3), lithium aluminium titanium phosphate (Li x Al y Ti z (PO 4) 3, 0<x<2, 0<y<1, 0<z<3), (LiAlTiP) x O y type glass (0<x<4, 0<y<13), lithium lanthanum titanate (Li x La y TiO 3, 0<x<2, 0<y<3), lithium germanium thiophosphate (Li x Ge y P z S w, 0<x<4, 0<y<1, 0<z<1, 0<w<5), lithium nitrides (Li x N y, 0<x<4, 0<y<2), SiS 2 type glass (Li x Si y S z, 0<x<3, 0<y<2, 0<z<4) and P 2 S 5 type glass (Li x P y S z, 0<x<3, 0<y<3, 0<z<7).