Microporous polypropylene hollow fibers with double layers were prepared by stretching double-layered polypropylene microtubes containing polymethylsilsesquioxane fillers : the relatively smaller filler in the inner layer and relatively larger filler in the outer layer. The resultant hollow fibers have a finely interconnected fibrous structure parallel to fiber axis. Their N-2 gas permeabilities were measured to estimate the fibrous structure : tortuosity factor, effective porosity, and pore size.