Process improvement practices (PIP) are being used as ways to improve manufacturing competitiveness (MC) but their overall effect in apparel factories have not been well quantified. This paper identifies the effects that PIP have on the manufacturing competitiveness of multinational apparel manufacturing plants. The methodology followed uses the Delphi method and exploratory factor analysis for variable selection, the Mann-Whitney test for analyzing the relationship between PIP usage and MC performance and regression analysis for estimating the existing correlation among the variables studied. The results show that factories with a high use of PIP have a higher manufacturing competitiveness and that specific practices related to error detection and waste elimination have a positive effect in cost, delivery time and environmental protection.