This research was done to study the effects of different fibre sizes (1 mm, 500 µm, 250 µm and 125 µm) on the mechanical, morphological and thermal properties of Sansevieria trifasciata fibre/natural rubber/high density polyethylene (STF/NR/HDPE) composites. Processing of STF/NR/HDPE composites was done by using an internal mixer machine. The processing parameters used were 135 °C for temperature and a mixing rotor speed of 55 rpm for 15 minutes. Filler loading was varied from 10% to 40% of STF. The composite blends obtained were pressed with a hot press machine to get test samples of 1 mm and 3 mm in thickness. Samples were evaluated via tensile tests, Izod impact test and differential scanning calorimeter (DSC). Morphological studies were carried out via scanning electron microscope (SEM). Results showed that tensile strength and impact strength decreased while tensile modulus increased when filler loading increased. Smaller particle size will give more interfacial interaction between fibre and matrix. Adding a higher amount of filler will also increase the viscosity and the stiffness of the materials. Overall, it showed that 125 µm of fibre size has given higher values/stable results of tensile strength and modulus. Thermal behaviour of the materials was not affected much by fibre size.