The present invention relates to a microporous polyolefin film suitable as a separator for batteries and thermal properties thereof. A microporous polyolefin film according to the present invention has a film thickness of 5-40 µm, a porosity of 30%-60%, a permeability of 2.0x10 -5 -8.0x10 -5 Darcy, a maximum pore size determined by the bubble point method of not more than 0.1 µm, a puncture strength of 0.20 N/µm or more at room temperature and 0.05 N/µm or more at 120°C, and a maximum shrinkage ratio in the transverse direction (TD) when subjected to a thickness-normalized external force in TMA (thermo-mechanical analysis) of not more than 0%. With excellent thermal stability at high temperature as well as superior puncture strength and gas permeability, the microporous polyolefin film according to the present invention is suitable for high-capacity,and high-power batteries.