The influence of fiber treatment on the properties of biocomposites derived from grass fiber and soy based bioplastic was investigated with environmental scanning electron microscopy, thermal and mechanical properties measurements. Grass fibers were treated with alkali solution that reduced the inter-fibrillar region of the fiber by removing hemicellulose and lignin, which reduce the cementing force between fibrils. This led to a more homogenous dispersion of the biofiber in the matrix as well as increase in the aspect ratio of the fiber in the composite, resulting in an improvement in fiber reinforcement efficiency. This led to enhancement in mechanical properties including tensile and flexural properties as well as impact strength. Additionally, the alkali solution treatment increased the concentration of hydroxyl groups on the surface, which led to a better interaction between the fibers and the matrix. (C) 2004 Published by Elsevier Ltd.