Compound of flax/polypropylene (PP) is characterized concerning the mechanical properties of stiffness, strength, and impact in addition to the water absorption behavior. Manufacturing takes place by twin-screw extruder. The extruder screw layout is modified through different kneading elements to get high fiber aspect ratio. Sodium hydroxide Solution was used as a washing solution for the flax fibers' Surfaces. Both fiber and matrix are chemically modified. Selected groups of the fibers were further treated using trimethoxyvinylsilan TMVS and acrylic acid AA. The PP matrix is also treated with different Coupling agents; namely, maleated PP MAPP, TMVS-MAPP, and acrylic acid-functionalized PP AAPP, The combinations of different fiber/matrix are extruder compounded, injection molded, and finally tested. Fiber modification seems to be positive with AA-modified surface. AAPP matrix modification improves the stiffness four times that of the untreated flax/PP. Till 30 and 40 wt %, the more the fiber is the more the strength and stiffness, respectively. MAPP-modified matrix improves the mechanical properties and keeps low water absorption values. AAPP-modified matrix shows the best stiffness values. TMVS-MAPP does not secin to have distinguished improvement compared with MAPP. NaOH-TMVS/MAPP and NaOH-TMVS/AAPP systems can serve as alternatives to the normal NaOH/MAPP treatment. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2279-2289, 2009