Banana stem fiber (BSF) reinforced low-density polyethylene (PE) composites were prepared with a hot-press molding machine in the presence of maleic anhydride grafted polypropylene (MAPP). To achieve better mechanical properties, the fiber was chemically modified by bleaching, alkalization, and acetylation. The ultimate tensile strength (UTS) of the untreated and treated BSF composites were found to increase with increasing fiber loading up to 20%, whereas the maximum Charpy impact strength (IS) and flexural strength (FS) values were seen at 10% fiber loading; these values decreased thereafter. The Young's modulus (YM) values of the BSF composites increased sharply with fiber loading. All of the treated fibers exhibited better mechanical properties than the untreated ones. The acetylated fiber showed higher UTS (44 MPa), FS (50 MPa), and IS (12.5 j/m(2)) values than the other treated and untreated fibers. The improvements in the mechanical properties of the treated composites were further supported by scanning electron microscopy images of the fracture surfaces. The thermal stabilities of the composites were studied by means of thermogravimetry, differential thermogravimetry, and differential thermal analysis measurements. Hybrid composites composed of BSF (10 wt %), coir fiber (5 wt %), and a MAPP/ low-density PE matrix were prepared. Significant improvements in UTS, YM, FS, and IS were seen in the hybrid composites containing surface-modified BSF. The effects of BSF composition on the composite properties were also studied. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128:1020-1029, 2013