The polymer composites of HDPE and banana, hemp, and agave fibers (50 : 50, w/w) were prepared separately with and without treatment of maleic anhydride. The swelling phenomenon in terms of absorption of water and steam was studied and it was found that the steam penetrates more within a smaller period of time than the water at ambient temperature. The maleic anhydride treatment on these fibers showed the ester-ification of fibers, and because of that, the absorption (swelling) of steam and water is less than the untreated respective fibers composites.