A crosslinked polyolefin separator having an average value of light transmittance of 30% or more in a region of 380 nm to 700 nm, after four sides of the separator are fixed and allowed to stand at 130° C. for 30 minutes. A method for manufacturing the crosslinked polyolefin separator is also provided. The crosslinked polyolefin separator has a low shutdown temperature to provide improved safety. The crosslinked polyolefin separator also has a high meltdown temperature and is inhibited from die-drooling.