Crystalline cellulose was extracted from one of the cheapest source; grass by alkali, acid, and mechanical treatments. To evaluate the reinforcing effect of this filter, biodegradable polymer matrix, polylactic acid (PLA) was used and samples were prepared at different concentrations. The modified whiskers were also used as filler to examine the effect on mechanical properties. Crystallinity, thermal behavior, surface morphology and functional group variation was monitored through, XRD, TGA, SEM, and FTIR respectively. The properties were largely depending on the concentration of filler whereas; modification of hydroxyl groups did not exhibit remarkable change in mechanical properties. Thermal resistance was decreased in comparison of neat matrix and reduction in crystallinity was also observed for composites, regardless of filler nature. It was thought that the hydrophilicity of cellulose Could be altered by modifying its hydroxyl groups, thereby promoting dispersion. However, the possibility of phase separation was increased, and the thermal stability of the composites gradually decreased with increasing filler load in the matrix. In the examined systems, it was observed that improvement of reinforcing capacity may not only depend on the chemical compatibility between matrixes but also some compositional and physical parameters may be responsible for variation in properties. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2493-2501, 2010