The present work studies single screw extrusion processing for different systems of natural fiber reinforced thermoplastics. In the present study the mechanisms of fiber organization and fiber degradation have been investigated. The polymer matrix employed has been high density polyethylene (HDPE) with an MFI of 0.1. Fibers including jute and sisal have been used as discrete reinforcements with initial lengths varying from 5 to 10 mm. The extrusion process has been investigated using an open-barrel single screw extruder. The issues of fiber distribution and bubble formation have been related to the mechanical properties of these composites, particularly tensile strength. Fiber degradation has been assessed after processing by microscopic examination of the extruder contents. The effect of the processing temperature on the level of fiber degradation is also discussed here. Fiber length after processing has been measured, and unimodal and bimodal fiber length distributions have been obtained. However, the issue of adequate sample preparation for the fiber length characterization procedure still requires further work in order to reduce the experimental error. Fiber orientation distributions have been estimated for some of the samples. It has been found that at higher processing temperatures the fibers show more alignment in the flow direction. © 2005 Wiley Periodicals, Inc.