Three types of polyethylene (PE), low-density PE (LDPE), linear low-density PE (LLDPE), and high-density PE (HDPE) were used as polymer matrices to prepare untreated as well as dicumyl peroxide (DCP) treated sisal fiber composites. The effect of polymer chain structure, addition of DCP, and sisal fiber loadings on the mechanical and dynamic mechanical properties of the composite was investigated in this study. It was found that the extent of improvement in tensile properties of the composite samples varied with respect to the polymer molecular characteristics. The elongation at break for all the composites decreased significantly. Young's modulus and the tensile strength of the treated LDPE and LLDPE composites increased significantly compared with the untreated composites, whereas Young's modulus of the treated HDPE samples decreased observably compared with the untreated samples. DCP treatment, however, did not change the tensile strength of HDPE and its composites. The storage modulus results for all the PE composites correlate well with the tensile testing results. In the case of the LDPE and LLDPE samples, the curves of the mechanical loss factor (tan d) show a clear relaxation around -18 degrees C, which shifted to higher temperature in the treated composites, whereas for HDPE this transition was not seen. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012