The experimentally observed tensile properties (tensile strength and modulus) of short sisal fibre-reinforced LDPE with different fibre loading have been compared with the existing theories of reinforcement. The macroscopic behaviour of fibre-filled composites is affected by fibre loading, orientation and length of the fibres in the continuous medium. The interfacial adhesion between fibre and matrix also plays a major role in controlling the mechanical properties of the fibre-filled composites. In this study, a comparison is made between experimental data and different theoretical models. Composite models, such as parallel and series, Hirsch, Cox, Halpin-Tsai, modified Halpin-Tsai and modified Bowyer and Bader, have been tried to fit the experimental data.