NOVELTY - Organic silicon microparticles consist of polysiloxane crosslinked structure component, and fusiform exhibited as whole having recessed portion on the surface of microparticles. The average value of the length and minimum shorter axis of fusiform is 0.01-20 mu m and 0.005-15 mu m, respectively. The average values are arbitrary average values of 100 pieces extracted from the scanning electron microscope image of organic silicon microparticles.    USE - Organic silicon microparticles are used for forming modified polyolefin microporous film used as nonaqueous electrolyte battery cell separator (all claimed) e.g. lithium ion battery cell separator. Can also be used for microfiltration membrane, separator for capacitor and fuel cell material. Uses include but are not limited to motor vehicles, mobile telephone and notebook-type personal computer.    ADVANTAGE - The organic silicon microparticles provides modified polyolefin microporous film having low heat shrinkage rate, even at high temperature and excellent shutdown function, shape stability and mechanical strength.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) manufacture of organic silicon microparticles, which involves hydrolyzing 20-50 mol% silanol-forming silicon compound of formula: SiX4 (IV), 20-70 mol% silanol-forming silicon compound of formula: R4SiY'3 (V) and 5-50 mol% silanol-forming silicon compound of formula: R5R6SiZ2 (VI) in presence of catalyst using mixed solvent of nitrogen compound and water, and performing condensation-reaction;    (2) modified polyolefin microporous membrane, which is obtained by forming a heat resistant porous layer made of organic binder and the organic silicone microparticles on both surfaces or one surface of microporous polyolefin membrane; and    (3) manufacture of modified polyolefin microporous film, which involves diluting the mixture of organic silicon microparticles and organic binder with solvent which dissolves the organic binder, preparing slurry, coating the prepared slurry on one surface or both surfaces of polyolefin microporous film base material, drying and forming.    X,Y',Z=1-4C alkoxy, alkoxyethoxy having 1-4C alkoxy, 2-4C acyloxy, N,N-dialkylamino having 1-4C alkyl, hydroxy, halo or H.