In the present work, PP‐based composites, reinforced with surface modified pine fibers, have been prepared. The surface of the fibers has been treated with several silane derivatives bearing specific functionalities. NH2, SH, long aliphatic chain, and methacrylic group were chosen as functionalities of the silane derivatives for evaluating the compatibility with the polymer matrix. Mechanical analysis, contact angle and XPS spectra, SEM microscopy, and water uptake measurements were used as characterization techniques for evaluating the nature of composites. XPS as well as contact angle measurements demonstrated that pine fibers and silane derivatives were effectively coupled. The mechanical analysis showed an increase in Young's and flexural moduli, by 12% and 130% respectively, and nonsignificant changes in the ultimate tensile strength were noted after surface modification. Water uptake measurements revealed a low water absorption by the materials, always lower than 2 wt %. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3706–3717, 2007