Cellulosic bioethanol produced from non-edible plants reduces potential food-fuel competition and, as such, is receiving increasing attention. In the raw material production of cellulosic bioethanol, the aboveground biomass of plants is entirely harvested; consequently, the plant roots represent the major source of organic matter incorporated into the soil. We selected Erianthus and Napier grass as the raw materials for cultivation in Asia. However, information about whether these 2 species provide sufficient root volume to sustain soil fertility is limited. Therefore, we examined the spatial distribution of the roots of these 2 plants, and quantified root mass and length. Erianthus and Napier grass were either grown in fields or greenhouses in Tokyo (Japan) and Lampung (Indonesia), and then their roots were exposed from adjacent soil profiles. Both species developed large, deep roots, penetrating 2.0-2.6 m deep into the soil. Root depth indexes showed that the roots of both species penetrated much deeper into the soil compared to monocot crop species, being more comparable to dicot species. Erianthus developed a root mass and length of 384-850 g·m^-2 and 28.8-35.8 km·m^-2, while the values for Napier grass were 183-448 g·m^-2 and 15.6-43.6 km·m^-2, respectively. These values exceeded the maximum values previously recorded for common crop species. Our study confirmed that Erianthus and Napier grass develop deep root systems, with substantially large biomass; hence, we suggest that both plants supply root biomass in large quantities, representing possible major sources of soil organic matter.