A separator of an electrochemical battery comprising: a) a porous substrate wherein the porous substrate is a polyethylene and having a thickness of ≥ μm to ≤ 40 μm; and (b) a heat resistant porous layer formed on one or both surfaces of the porous substrate using a composition comprising: at least one selected from the group consisting of a monomer, oligomer, and polymer and a mixture thereof, wherein the monomer is represented by Formula 1, and the oligomer and polymer are polymerized from the monomer; at least one selected from the group consisting of a polyvinylidene fluoride (PVdF)-based homopolymer, a polyvinylidene fluoride-hexafluoropropylene-based copolymer, and a mixture thereof; an initiator, wherein the initiator include thermal polymerization initiators comprising a peroxide, azo, amine, imidazole, or isocyanate initiators; and a solvent, wherein in the polyvinylidene fluoride-hexafluoropropylene-based copolymer, a unit originating from hexafluoropropylene is present in an amount of > 0 wt% to ≤ 15 wt% based on the total weight of the polyvinylidene fluoride-hexafluoropropylene-based copolymer, and the polyvinylidene fluoride-hexafluoropropylene-based copolymer has a weight average molecular weight of ≥ 600,000 g/mol; where X 1 to X 3 are each independently an oxyethylene group; X 4 is an oxyethylene group or a C 1 to C 10 alkyl group; R 1 to R 4 are each independently any one selected from the group consisting of a (meth)acrylate group, a hydroxy group, a carboxyl group, an ester group, a cyanate group, an isocyanate group, an amino group, a thiol group, a C 1 to C 10 alkoxy group, a vinyl group, and a heterocyclic group; a 1 to a 4 are each independently an integer from 1 to 10; n 1 to n 3 are each independently an integer from 0 to 10; and at least one of n 1 to n 4 is an integer from 1 to 10, and when X 4 is an oxyethylene group, n 4 is an integer from 1 to 10 and m is 1, and when X 4 is a C 1 to C 10 alkyl group, n 4 is 1 and m is 0. a) a porous substrate wherein the porous substrate is a polyethylene and having a thickness of ≥ μm to ≤ 40 μm; and (b) a heat resistant porous layer formed on one or both surfaces of the porous substrate using a composition comprising: at least one selected from the group consisting of a monomer, oligomer, and polymer and a mixture thereof, wherein the monomer is represented by Formula 1, and the oligomer and polymer are polymerized from the monomer; at least one selected from the group consisting of a polyvinylidene fluoride (PVdF)-based homopolymer, a polyvinylidene fluoride-hexafluoropropylene-based copolymer, and a mixture thereof; an initiator, wherein the initiator include thermal polymerization initiators comprising a peroxide, azo, amine, imidazole, or isocyanate initiators; and a solvent, wherein in the polyvinylidene fluoride-hexafluoropropylene-based copolymer, a unit originating from hexafluoropropylene is present in an amount of > 0 wt% to ≤ 15 wt% based on the total weight of the polyvinylidene fluoride-hexafluoropropylene-based copolymer, and the polyvinylidene fluoride-hexafluoropropylene-based copolymer has a weight average molecular weight of ≥ 600,000 g/mol; where X 1 to X 3 are each independently an oxyethylene group; X 4 is an oxyethylene group or a C 1 to C 10 alkyl group; R 1 to R 4 are each independently any one selected from the group consisting of a (meth)acrylate group, a hydroxy group, a carboxyl group, an ester group, a cyanate group, an isocyanate group, an amino group, a thiol group, a C 1 to C 10 alkoxy group, a vinyl group, and a heterocyclic group; a 1 to a 4 are each independently an integer from 1 to 10; n 1 to n 3 are each independently an integer from 0 to 10; and at least one of n 1 to n 4 is an integer from 1 to 10, and when X 4 is an oxyethylene group, n 4 is an integer from 1 to 10 and m is 1, and when X 4 is a C 1 to C 10 alkyl group, n 4 is 1 and m is 0. at least one selected from the group consisting of a monomer, oligomer, and polymer and a mixture thereof, wherein the monomer is represented by Formula 1, and the oligomer and polymer are polymerized from the monomer; at least one selected from the group consisting of a polyvinylidene fluoride (PVdF)-based homopolymer, a polyvinylidene fluoride-hexafluoropropylene-based copolymer, and a mixture thereof; an initiator, wherein the initiator include thermal polymerization initiators comprising a peroxide, azo, amine, imidazole, or isocyanate initiators; and a solvent, wherein in the polyvinylidene fluoride-hexafluoropropylene-based copolymer, a unit originating from hexafluoropropylene is present in an amount of > 0 wt% to ≤ 15 wt% based on the total weight of the polyvinylidene fluoride-hexafluoropropylene-based copolymer, and the polyvinylidene fluoride-hexafluoropropylene-based copolymer has a weight average molecular weight of ≥ 600,000 g/mol; where X 1 to X 3 are each independently an oxyethylene group; X 4 is an oxyethylene group or a C 1 to C 10 alkyl group; R 1 to R 4 are each independently any one selected from the group consisting of a (meth)acrylate group, a hydroxy group, a carboxyl group, an ester group, a cyanate group, an isocyanate group, an amino group, a thiol group, a C 1 to C 10 alkoxy group, a vinyl group, and a heterocyclic group; a 1 to a 4 are each independently an integer from 1 to 10; n 1 to n 3 are each independently an integer from 0 to 10; and at least one of n 1 to n 4 is an integer from 1 to 10, and when X 4 is an oxyethylene group, n 4 is an integer from 1 to 10 and m is 1, and when X 4 is a C 1 to C 10 alkyl group, n 4 is 1 and m is 0.