A separator, a method of manufacturing the same, and a lithium secondary battery including the same are disclosed herein. In some embodiments, a separator includes a non-crosslinked polyolefin layer; and a crosslinked polyolefin layer disposed on one surface of the non-crosslinked polyolefin layer and having at least one crosslinking bond represented by the following Chemical Formula 1, wherein the separator is configured such that the non-crosslinked polyolefin layer of the separator faces a positive electrode. In some embodiments, a lithium secondary battery includes a positive electrode, a negative electrode and the separator interposed between the positive electrode and the negative electrode. The lithium secondary battery has a high melt-down temperature and shows high oxidation stability under high-voltage/high-temperature environment.