The present study deals with the effects of natural fibers on thermal and mechanical properties of natural fiber polypropylene composites using dynamic mechanical analysis. Composites of polypropylene and various natural fibers including kenaf fibers, wood flour, rice hulls, and newsprint fibers were prepared at 25 and 50% (by weight) fiber content levels. One and two percent maleic anhydride grafted polypropylene was also used as the compatibilizer for composites containing 25 and 50%, fibers, respectively. Specimens for dynamic mechanical analysis tests were cut out of injection-molded samples and were tested over a temperature range of -60 to +120 degrees C. Frequency of the oscillations was fixed at 1 Hz and the strain amplitude was 0.1%, which was well within the linear viscoelastic region. The heating rate was 2 degrees C/min for all temperature scan tests. Storage modulus (U), loss modulus (U), and mechanical loss factor (tan 6) were collected during the test and were plotted versus temperature. An increase in storage and loss moduli and a decrease in the mechanical loss factor were observed for all composites indicating more elastic behavior of the composites as compared with the pure PP. Changes in phase transition temperatures were monitored and possible causes were discussed. Results indicated that glass transition was slightly shifted to lower temperatures in composites. alpha transition temperature was higher in the case of composites and its intensity was higher as well. (c) 2006 Wiley Periodicals, Inc.