The water bamboo husk is one of major agricultural wastes in Taiwan. In this study, the powder obtained from the water bamboo husk was added to poly(lactic acid) (PLA) to form novel reinforced biodegradable composites. Morphologies, mechanical properties, and heat resistance of these water bamboo powder reinforced composites were investigated. The results indicate that the char yields were increased as plant powder was incorporated to PLA. In addition, the mechanical properties were also enhanced due to the addition of powders. The increments of storage moduli of PLA were about 50-200%. Moreover, the increments of loss moduli of PLA were about 70-200%. On the other hand, the T-g of PLA was slightly decreased by the addition of powder, and this may improve the brittle characteristics of PLA. Furthermore, this type of reinforced PLA would be more environmental friendly than the artificial additive-reinforced one.