Polypropylene fibers having improved heat-shrinkability and tenacity

Polypropylene fibers having an improved heat-shrinkability and tenacity, which comprises a polypropylene resin having a density of 0.905 g/cc or more, an isotactic pentad ratio of boiling n-heptane-insoluble portion (P.sub.0) of 0.960 or more and a ratio of pentad having two different kinds of configurations (P.sub.2), of 0.002 (0.2%) or less prepared by the process of polymerizing propylene in the presence of a catalyst prepared by reacting an organoaluminum compound or a reaction product of an organoaluminum compound with an electron donor, with TiCl.sub.4, further reacting the resulting solid product with an electron donor and an electron acceptor, and them combining the resulting solid product with an organoaluminum compound and an aromatic carboxylic acid ester, the molar ratio of said aromatic carboxylic acid ester to said solid product being in the range of 0.2 to 10.0.