Using rice straw, bagasse, and cotton stalk fibers as reinforcing fillers in polyester composite was studied. The effects of fiber loading and fiber size on the modulus of rupture (MOR), modulus of elasticity (MOE), tensile strength, water absorption, and thickness swelling were investigated as well. The effect of esterification. using maleic anhydride on the aforementioned properties also was studied. Ultraviolet light resistance and thermal behavior of the produced composites were also investigated.