In the present study both treated and untreated sisal polyester composites are tested to study its impact strength characteristics. ASTM D256 norms are followed to conduct the impact test on the specimens of 2 mm, 3 mm, 4 mm, 5 mm and 6 mm thicknesses for fiber volume of 10%, 15%, 20%, 25% and 30%. Random orientation of fibers was adopted. Length of fibers used in casting the specimen is as 10 mm. Here all the specimens are fabricated by using manually operated hot compression moulding technique. The results obtained from the present study have shown that impact strength increases with increase in the thickness. Untreated sisal polyester composite yielded its peak impact strength of 3.581 N-m at its 30% fiber volume fraction. Treated sisal polyester composite has shown highest impact strength of 1.962 N-m at its 30% fiber volume fraction. © 2017 The Authors.