In the present study an attempt is made to determine the mechanical properties of sisal fiber reinforced polyester composites. Sisal fibers are the natural fibers obtained by processing the leaves of the sisal plants grown in nature. Sisal plant offers hard and strong strands of sisal fibers. The soft tissue of the sisal leaves is removed either physically or by using equipments. The fibers obtained are dried and brushed to remove the dirt left over to get the sisal fibers. In the present study, randomly oriented sisal fiber reinforced polyester matrix composite specimens of thicknesses 2 mm, 3 mm, 4 mm, 5 mm and 6 mm were fabricated by using hot compression moulding technique. 5% NaOH treated sisal fibers of length 10 mm is used as reinforcement for casting the composite specimens. A mixture of polyester resin, methyl ethyl ketone peroxide and cobalt naphthenate of ratio 50:1:1 is used as matrix for the fabrication of composite panels. Composite panels of fiber volume fraction 10%, 15%, 20%, 25% and 30% were casted and the test specimens were cut from the panels and tested for its tensile strength and flexural strength as per ASTM D-3039 and ASTM D-7264 respectively. From the experimental results it is observed that strength of tested specimens was found to show peak values at a fiber volume fraction of 20–25%. © 2018, Springer International Publishing AG.