For the purpose of improving the toughness of natural fiber green composites, mercerization was applied for ramie fibers. Mercerization, i.e. alkali treatment to single-fibers and fiber bundles of ramie, was carried out by NaOH solution, and the biodegradable resin matrix composite using the mercerized fiber bundles was press-formed. The results of tensile test showed that both the mercerized single-fibers and bundles were improved in tensile strength slightly higher than the untreated fibers, while fracture strains of the mercerized single-fibers and bundles increased to 0.066 and 0.060, respectively, i.e. approximately twice higher than those of the untreated fibers. From such a change in fracture strain, the composite using the mercerized fiber bundles was largely improved in plastic deformation function.