Disclosed is a separator for a lithium secondary battery including a porous polyolefin substrate and showing a tan(δ) of 0.3 or less in the storage-loss modulus curve. Also disclosed is an electrochemical device including the separator. The separator shows low viscosity but high elasticity at high temperature. Therefore, the separator maintains its strength at high temperature and ensures resistance against external force in the state of high-temperature exposure, and thus can provide a lithium secondary battery with improved safety. In addition, according to an embodiment of the present disclosure, it is possible to obtain a crosslinked polyolefin separator through a simplified process, unlike the related art.