Composites of polypropylene/natural fiber were obtained synthesizing polypropylene in the presence of chemically treated fibers. For this, vegetal cellulose fibers were introduced in the polymerization medium of propylene using Ziegler-Natta catalyst. The fibers were treated to increase their compatibility with the polypropylene matrix and decrease the accessibility of functional groups to the catalyst active sites. At first, mild acid hydrolysis was performed, followed by chemical treatments with triethylaluminum and stearic alcohol. The results showed that the chemical treatments increased the crystallinity index to 73-77% in comparison with the original fiber (57.6%). The catalyst activity and its stereospecificity maintained high when fiber was treated with triethylaluminium and that with stearic alcohol. The composites contained amounts of fiber in a range of 3.1-5.8%. The fibers did not induce crystallization of PP crystallites different from the most common -form. Fibers were fibrillated to nanometric diameter, although micrometric length. POLYM. ENG. SCI., 56:71-78, 2016. (c) 2015 Society of Plastics Engineers