The use of plant fibers as a reinforcement in polyester matrices requires the issue of compatibility between the two phases to be addressed. Because plant fibers present hydrophilic surfaces and polyesters are generally hydrophobic, poor fiber-matrix dispersion and wetting of the fibers by the matrix may result. As a consequence, the mechanical properties of the composite are severely reduced. This study considers the effect of fiber treatment by chemical modification of the fibers (acetylation) or the use of silane or titanate coupling agents on the mechanical properties of coir or oil palm reinforced polyester composites.