The effect of chemical treatment on the tensile properties of sisal fibre-reinforced LDPE (low density polyethylene) composites was investigated. Treatments using chemicals such as sodium hydroxide, isocyanate, permanganate and peroxide were carried out to improve the bonding at the fibre-polymer interface. The treatments enhanced the tensile properties of the composites considerably, but to varying degrees. The SEM (scanning electron microscopy) photomicrographs of fracture surfaces of the treated composites clearly indicated the extent of fibre-matrix interface adhesion. It has been demonstrated that the CTDIC (cardanol derivative of toluene diisocyanate) treatment reduced the hydrophilic nature of the sisal fibre and thereby enhanced the tensile properties of the sisal-LDPE composites. The SEM photomicrographs of the fracture surfaces have also shown that PE was highly bonded to the sisal fibre in CTDIC treated composites. The observed enhancement in tensile properties with the addition of small amounts of peroxides was attributed to the peroxide induced grafting of PE on to sisal fibre surfaces, as evident from the SEM photomicrographs of the fracture surfaces. It has been found that a low concentration of permanganate in the sisal-LDPE system during mixing considerably enhanced the mechanical properties. Among the various treatments, peroxide treatment of fibre imparted maximum interfacial interactions.