The principle of capillary flow porometry and its application in the pore size distribution of microporous separator are introduced．The polypropylene microporous separator was used，and the obtained pore size is near that obtained by scanning electrical microscopy．Compared with mercury intrusion porosimetry，capillary flow porometry has such advantages as no use of toxic liquid，low test pressure，and with only the throat diameter of through poretested，no damage to samples．Th erefore，it is more useful in the structure characterization of mieroporous separators．