Microporous polyolefin film and method for preparing the same

Provided are a microporous polyolefin film usable as a separator for a lithium ion secondary battery and a method for preparing the same, the microporous polyolefin film including one or more layers, wherein the microporous polyolefin has a thickness of 5-40 µm, a porosity of 30¼60%, a puncture strength of 0.22N/µm or higher, and a quality factor of 20gf/(sec/100cc) or greater, so that the microporous polyolefin film can have excellent mechanical strength, permeability, and heat shrinkage characteristics, and thus, is suitably used for a separator for a high-capacity and high-output battery.