Styrene-divinylbenzene or glycidylmethacrylate-divinyl-benzene were copolymerized in powdery polypropylene suspended in water and the resultant polymer composites were blended with a definite amount of polypropylene. The products consisted of polypropylene and the fine particles of the copolymer, which were uniformly dispersed in polypropylene phase. These products were melt-spun to prepare polypropylene fibers containing the fine particles and then the fibers were stretched to make the fibers microporous. Some properties were estimated: porosity, 1.6-19.7%; average pore size, 0.004-0.009 mu m; and specific surface, 9-137 m(2)/g.