To determine the optimum $ extrm{P}_2 extrm{O}_5$ rate for dry matter production of Achyranthes japonica Nakai a medicinal plant, in Jeju island, plants were grown in 2001 under ten fertilizer treatments comprising of two N (90, and 180kg/ha) and five $ extrm{P}_2 extrm{O}_5$ (0, 100, 200, 300, and 400 kg/ha) levels. Plant height leaf length and width, stem diameter, and main root length and diameter were not significantly affected by $ extrm{P}_2 extrm{O}_5$ rate while $ extrm{P}_2 extrm{O}_5$ fertilization of 100 kg/ha increased the number of branches, spikes, and roots per hill by 18,38 and 43%, respectively, compared with the unfertilized $ extrm{P}_2 extrm{O}_5$ treatment and decreased with further increases in $ extrm{P}_2 extrm{O}_5$ rate. The number of roots per hill was greatest with the application of 200kg $ extrm{P}_2 extrm{O}_5$. The highest utricle dry matter yield (2.92 t/ha) was obtained with the application of 100kg $ extrm{P}_2 extrm{O}_5$. However, as $ extrm{P}_2 extrm{O}_5$ rate increased from 0 to 100 and 200kg/ha, root dry matter yield increased from 2.36 to 3.55, and 3.80 t/ha, and then decreased to 3.14, and 2.86 t/ha at 300, and 400kg $ extrm{P}_2 extrm{O}_5$ rates, respectively. As $ extrm{P}_2 extrm{O}_5$ rate increased from 100 to 400 kg/ha, $ extrm{P}_2 extrm{O}_5$ recovery in aboveground parts decreased from 34.1 to 7.1%.