The effect of chemical modification of both fiber and matrix on melt rheological behavior of intimately mixed short sisal-glass hybrid fiber-reinforced low-density polyethylene composites was studied with an Instron capillary rheometer. The variations of melt viscosity with different shear rate and shear stress values for different temperatures were studied. A temperature 1 range of 130 to 150degreesC and shear rates of 16.4 to 5468 s(-1) were chosen for the analysis. Chemical modifications with stearic acid, maleic anhydride, silane, and peroxides were tested for their ability to improve the interaction between the matrix and fiber. The viscosity of the hybrid composites increases with every chemical modification. In the case of peroxide-treated composites, the increase can be attributed to the peroxide-induced grafting of the polyethylene matrix to the fiber surface and to the crosslinking of the polyethylene matrix. These phenomena are both activated by temperature, whereas temperature causes a reverse effect for all other chemical modifications. (C) 2003 Wiley Periodicals, Inc.