In current time, fracture toughness of biomass based plastics that consist of poly (latic acid), PLA and kenaf short fibers, which absorbs its CO_2 efficiently is evaluated. Adding this fiber to PLA greatly increase its fracture toughness. Compared with the precedence research, we got the result a little better than precedence research. From these results, increase of the mechanical property by kenaf content is certainly observed, and application for a structural material use is shown to be possible.