ABSTRACT Environmental awareness and an increasing concern with sustainable development have stimulated many industries including ground engineering to replace the conventional synthetic fibers. In this work, flexural strength and flexural modulus of chemically treated random short sisal fiber reinforced polypropylene and unsaturated polyester composites were investigated over a range of fiber content. Flexural strength of the composites was found to decrease with increased fiber content; however, flexural modulus increased with increased fiber content. The reason for this decrease in flexural strength was found to be due to fiber defects which could induce stress concentration points in the composites during flexural test, accordingly flexural strength decreased. Alkali and silane fiber treatments were found to improve flexural strength and flexural modulus which could be due to enhanced fiber/matrix adhesion.