Abstract: The composition comprises polyolefins, fillers and a plasticizer. It contains 0.2 to 20 wt. % etoxysiloxane, 5 to 60 wt. % polyolefin, 10 to 60 wt. % white black filler, and 10 to 75 wt. % polyethyleneglycol, petroleum oil or dioctylphtalate. The polyolefin can be high-density polyethylene with a melt index at a standard charging up to 6, superhigh-molecular polyethylene, polypropylene, or a mixture of polypropylene and polyethylene, or of high-density polyethylene and super-high-molecular polyethylene at as ratio of 0 to 100 wt. parts high-density polyethylene for every 100 wt. parts super-high-molecular polyethylene. The composition is prepared by treating the filler with etoxysiloxane at room temperature, then adding the polyolefin and the plasticizer to form a mixture, and then homogenizing the mixture in a cold state or with heating. With this composition are prepared microporous separators having an average pore size of below 0.05.mu., and having high physicomechanical properties.