This article focuses on the reinforcement of hydrogenated acrylonitrile butadiene rubber (HNBR) by cotton fiber as natural reinforcing filler. The effect of fiber alignment on the properties of HNBR compounds and vulcanizates is investigated. Properties of interest include rheological behavior, cure, tensile, abrasion, and dynamic mechanical properties which are correlated to the magnitudes of state-of-mix, bound rubber content, crosslink density and fiber alignment. Results obtained reveal that mechanical properties of rubber composites are improved dramatically by the addition of cotton fiber due to the enhanced hydrodynamic effect in association with crosslink density. Furthermore, the degree of fiber alignment is found to depend strongly on shear strain. The results demonstrate the importance of fiber alignment controlled efficiently by shear strain. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41090.