In the present work, banana, hemp, and sisal fibers are employed as fillers for the formation of natural fiber: polystyrene composites in the ratios of 55:45, 50:50, 45:55, and 40:60 (wt/wt). The natural fibers were treated with maleic anhydride. The Young's modulus, flexural modulus, impact strength, and Shore-D hardness all decreased with increasing amounts of fibers in the natural fiber: polystyrene composites. The sisal fiber composites show the highest mechanical strength for all ratios. A maleic anhydride treatment shows significant improvement in Young's modulus, flexural modulus, impact strength, and Shore-D hardness compared with the untreated fiber composites.