Natural fibers are available in nature as byproducts of agricultural products of various countries around the world. It is observed that coir fibers is product obtained from coconut but jute and sisal fibers are obtained from plants grown in nature. These fibers are very abundantly available in nature can be used for structural construction practices. This increases the economic value of these fibers. In the present study an effort is made to study the mechanical properties of sisal fiber reinforced composite materials. 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. Untreated 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. Each composite panels of fiber volume fraction 10 %, 15 %, 20 %, 25 % and 30 % were 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 tensile strength and flexural strength were increasing up to 20 % fiber volume fraction and further found to be decreasing for fiber volume fractions of 25 % and 30 %. But in case of specimens of 6 mm thickness a small change in the trend of results is observed. The increase in tensile strength is found to be continuous up to 30 % of fiber volume fraction with a tensile strength of 22.938 MPa at 30 % fiber volume fraction. © The Society for Experimental Mechanics, Inc. 2017.