The melt rheological behavior of neem bark flour (BF) filled high density polyethylene (HDPE) has been studied at varying volume fraction (Phi(f)) from 0 to 0.26 at 180, 190, and 200 degrees C in the shear rate range from 100 to 5000 s(-1) using extruded pellets of the composites. The melt viscosity of HDPE increases with Phi(f) because the BF particles obstruct the flow of HDPE. With the incorporation of the coupling agent HDPE-g-MAH, the viscosity decreased compared to the corresponding compositions in the HDPE/BF systems due to a plasticizing/lubricating effect by HDPE-g-MAH. The composites obeyed power law behavior in the melt flow. The power law index decreases with increase in the filler content and increases with temperature for the corresponding systems while the consistency index showed the opposite trend. The activation energy for viscous flow exhibited inappreciable change with either Phi(f) or inclusion of the coupling agent, however, the pre-exponential factor increased with filler concentration. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 2122-2130, 2012