Rice is a typical silicon accumulating plant, which accumulates up to 10% silicon in the shoot. Silicon absorbed by rice provides many benefits including improvements in pest and pathogen resistance, heavy-metal tolerance, and rice quality and yield. Silicon is absorbed by plants as uncharged silicic acid, Si(OH)₄, and it is ultimately precipitated irreversibly throughout the plant as amorphous silica, SiO₂-nH₂O, referred to as phytoliths. Amorphous silica is precipitated first on the small cells, as the short cells, and next on the larger cells, such as the motor cells or trichomes (Kondo and Sato, 1986). Phytoliths have been considered to be fairly stable, however, it is also considered as important source of silicon to rice plant. A good correlation exists between the silicon content of rice straw and soil soluble silicon measured by the incubation methods under submerged conditions (Takahashi and Nonaka, 1986). In this study, we investigated phytolith composition of rice leaf blade and effects of long-term application of rice straw on plant-available silicon of paddy soil. Rice leaf blades with different silicon concentration were taken at the maximum tiller number stage. Phytoliths were collected from leaf blade after decomposition by the solution of H₂SO₄ – HCl – HNO₃ and were observed under optical microscope. The pair samples of topsoil and subsoil treated with or without rice straw were collected from Aizu, Inawashiro, Koriyama, and Soma Agricultural Experiment Station in Fukushima prefecture. Soils collected are classified as Fluvisols. Application rate of rice straw in Aizu, Koriyama, and Soma was 6.0 Mg ha^-1, and 5.0Mg ha^-1 in Inawashiro. In Aizu, silicate fertilizer was applied. With or without rice straw applications has been practiced since 1920 in Aizu; 1954 in Inawashiro; 1985 in Koriyama; and 1998 in Soma. To determine plant-available silicon in the soil, 10 g of air-dried sample with 60 ml distilled water was incubated at 40C for one week (Takahashi and Nonaka, 1986). The phytoliths of rice leaf blade are composed of silicified short cells, motor cells and trichome. The amounts of silicified short cell and motor cell were 1.0 - 2.0 x 10⁷ g^-1DW and 1.0 - 25 x 10⁴ g^-1DW respectively. There was exponential relationship between number of silicified motor cell and silicon content of rice leaf blade, while no clear relationship between number of silicified short cells and silicon content of leaf blade derived from this experiment. The expected numbers of silicified silica cell calculated from the amounts of rice straw applied in Aizu and Inawashiro topsoils were much smaller than those of actually observed one, while the expected numbers of silicified motor cells were close to those of observed one. Therefore, parts of silicified short cell might be dissolved during long-term experiment. Silicon concentration of supernatant collected from the incubated soil ranged from 11.2 to 18.2 mg L^-1 in the topsoils with rice straw in long-term experiments and from 10.9 to 17.4 mg L^-1 in their subsoils. These values are more than the solubility of biogenic opal (0.5 – 10 mg L^-1) reported by Drees et al (1989). Therefore, the plant available silicon in paddy soils with rice straw may originate not only from phytoliths of rice straw, but also from more soluble silicon precipitation contained in rice straw and/or diatomaceous debris in the paddy soil. Total silicon input from rice straw applied from long-term experiment in Aizu, Inawashiro, Koriyama and Soma stations were calculated as 5.04, 2.50, 1.14, and 0.36 Mg Si ha^-1 respectively with an assumption that average silicon content of rice straw is 10 %. These high inputs of silicon must affect on plant available silicon in the paddy soil with or without rice straw. Actually, plant available silicon in the topsoil of Aizu, Inawashiro, Koriyama and Soma stations applied with rice straw were 109, 114, 77.2 and 70.2 mg Si kg^-1, while those without rice straw were 39.4, 87.6, 76.4 and 55.8 mg Si kg^-1 respectively. The extremely large difference between Aizu soils with and without rice straw might be the result of silicate fertilizer application. Plant available silicon in their subsoil from the each plot with rice straw were 86.4, 83.0, 105 and 68.0 mg Si kg^-1 respectively. These values were 1.1 – 1.6 times higher than those without rice straw. This means parts of silicon dissolved from rice straw in topsoil may easily move into subsoil in paddy fields condition, especially in Koriyama station field, where only small difference was observed in the plant available silicon of topsoil with or without rice straw, but large difference in subsoil. Therefore, we should pay attention not only plant available silicon in topsoil but also that in subsoil when we estimate silicon fertility of paddy soil.