Disclosed herein is multi-layer, microporous polyolefin membrane comprising a first porous layer comprising primarily a polyethylene, and a second porous layer comprising a polyethylene resin and polypropylene, the polypropylene having a weight-average molecular weight of 6.times.10.sup.5 or more and a heat of fusion (measured by a differential scanning calorimeter) of 90 J/g or more, a fraction of the polypropylene having a molecular weight of 5.times.10.sup.4 or less being 5% or less by mass.