Disclosed is a microporous polyethylene film for a battery separator and a method of producing the same. The microporous polyethylene film comprises a resin mixture, which includes 100 parts by weight of composition containing 20-50 wt % polyethylene with a weight average molecular weight of 5.times.10.sup.4-3.times.10.sup.5 (component I) and 80-50 wt % diluent (component II), 0.1-2 parts by weight of peroxide (component III), and 0.05-0.5 parts by weight of anti-oxidant (component IV). The microporous polyethylene film has a puncture strength of 0.22 N/.mu.m or more and a gas permeability (Darcy's permeability constant) of 1.3.times.10.sup.-5 Darcy or more. The present invention increases production efficiency of the microporous film, and improves performances and stability of the battery when the microporous polyethylene film is used in a battery separator.