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 where one or more layers are laminated includes a polyethylene layer having a maximum pore diameter of 0.1 [mu]m, wherein the microporous polyolefin has a thickness of 5-40 [mu]m, a porosity of 30-60%, a puncture strength of 0.22N/[mu]m 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.