This work describes a systematic study of the compounding conditions by twin-screw extrusion on the defibrization process that modulates the fiber aspect ratio and in turn the mechanical properties. Composites made of polycaprolactone reinforced by 20% hemp fibers were prepared by melt blending. The influence of the extrusion parameters (screw speed, feed rate, barrel temperature, and screw profile) and the initial fiber moisture content on both the fiber dimensions and the mechanical performance of the composite was investigated. The fiber aspect ratio increased when the fibers were water plasticized, principally at a higher feed rate and under a moderated extrusion temperature. The screw speed slightly influenced the fiber dimensions. Flow modeling was used to estimate the specific mechanical energy provided to the fibers, which ranged from 300 to 1700 kWh/t. Independent of the screw profile, a decrease in fiber length with an increase in energy was observed. The evolution of the fiber length and aspect ratios with respect to energy can be accurately described by an exponential function. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 1227-1238, 2013