Nowadays, the awareness of the public along with strict legitimate forces over the use of polymers, the manufacturing and automotive industries started using the renewable materials. Since, natural fiber reinforced composites play vital role in developing lightweight structural materials, this study focuses on utilizing sisal fiber as reinforcement in polyester matrix along with natural filler. The influence of fiber length and fiber volume fraction on the mechanical properties of sisal fiber was studied initially. Test results revealed that the composite with 20 mm fiber length and 20-volume fraction composite has better mechanical properties. Furthermore, the effect of fiber surface modification has been analyzed using various chemical solutions such as NaOH, KMnO4, stearic acid, and maleic acid. Of these, NaOH treatment enhances the mechanical properties of composite compared to all other cases. Finally, the influence of Acacia nilotica, a natural filler addition into the alkali-treated sisal fiber composite has been evaluated by mechanical and dynamic mechanical properties. It is found that the addition of natural filler and surface treatment has enhanced the properties of composites due to their synergetic effect. This effect improves the adhesion and uniform stress transfer among the reinforcements. The fiber surface morphology was evaluated using micrographs obtained from scanning electron microscope. © 2017 Taylor & Francis Group, LLC.