Abstract The influence of nanosized filler particles on mechanical properties of composite fibers has some specific features related to the fiber matrix, its preparation and the filler content, as well as the orientation of the fibers at their drawing. This paper discusses the manner of dispersion of chemically unmodified boehmite type nanofiller, Disperal 40 (D40) or Disperal 60 (D60) in polypropylene (iPP) fibers, and the influence of filler and its particle size on the tenacity, modulus and elongation of the iPP fibers. It was found that filler D40 was effective in increasing the tenacity only up to a content of about 1 wt% and in fibers prepared only at higher drawing ratios. Transmission electron microscopy (TEM) pictures of the cut iPP+D40 or iPP+D60 fibers have shown the presence of aggregation of nanosized boehmite particles in all cases. A comparison of the effects of two different particle sized fillers on the mechanical properties of the isotactic PP (iPP) fibers showed that a small difference between the particle sizes of both kinds of filler plays some role regarding the tensile strength of the fibers. The iPP fibers containing a mixture of D40 and D60 showed some deviations from the expected sum of the effects of both fillers on their mechanical properties.