Polyurethane (PU) based on castor oil and hexamethylene diisocyanate (HDI) was synthesized by a two-step method in the presence of dibutyltin dilaurate (DBTDL) as catalyst. The PU obtained was blended with various amounts of cellulose fibers extracted from alfa stems to prepare composite materials. The influence of cellulose fibers on the thermal and mechanical properties of different composites was demonstrated by means of several characterization techniques, such as thermomechanical, structural, and morphological analyses. The hydrogen bonding interaction and the possible formation of urethane links between PU and hydroxyl groups of cellulose provide a good fiber-matrix interface. The infrared absorption vibration of the N-H bond, which is 3336 cm(-1) for pure PU, in fact shifts to 3319 cm(-1) for a composite containing 25% of cellulose. Moreover, the variation of glass transition temperature, T-g, versus cellulose content is linear, for example, T-g of pure PU and of a composite containing 40% of cellulose is -35 and -24 degrees C, respectively. Furthermore, the mechanical properties rise steadily up to 20 wt % of fibers and decrease beyond this value. The novelty of this article lies in the fact that over 95% of starting materials used are obtained from agro-resource. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 902-909, 2010