The purpose of this study is to improve the strength and stiffness of natural fiber green composites through mechanical treatment, and to equalize the mechanical properties of the composites with those of glass fiber composites. Cyclic tensile stress at 70% level of their initial strengths was applied twenty times for single ramie fibers and ramie fiber green composites. The tensile strengths and Young's moduli of the fibers and composites were improved 50 to 60% higher than their initial values. This effect is also exhibited for mercerized ramie fibers and mercerized ramie fiber green composites. On the other hand, a large fracture strain, the merit of mercerization, was reduced because of occurrence of plastic strain in the mercerized fibers. Young's modulus of the improved composites is comparable to the level of glass fiber composites.