Natural fiber-thermoplastic composite materials, based on their cost-effectiveness and environmental friendliness, have attracted much interest both scientifically and technologically in recent years. Other advantages of natural fibers are good specific strength, less abrasion, and less irritation upon inhalation (in comparison with some common inorganic fillers). In the present contribution, roselle (Hibiscus sabdariffa L.) fibers were chosen and used as reinforcing fillers for isotactic polypropylene (iPP) for the first time, due mainly to the cost-effectiveness and natural abundance on Thai soil. Processibility and mechanical properties of the resulting composites were investigated against the type and the mean size of the fibers. The results showed that the highest mechanical properties were observed when roselle bast fibers were incorporated. When whole-stalk (WS) fibers (i.e., the weight ratio of bast and core fibers was 40 : 60 w/w) were used, moderate mechanical properties of the resulting composites were realized. The optimal contents of the WS fibers and the maleic anhydride-grafted iPP compatibilizer that resulted in an improvement in some of the mechanical properties of the resulting composites were 40 and 7 wt %, respectively. (c) 2006 Wiley Periodicals, Inc.