Roots play important roles in crop adaptation to water stress but the root traits responsible for the adaptation have not been clearly identified. This paper aims to review recent progress in our research on functional roles of the important root traits. We have been paying special attention to drought as well as soil moisture fluctuation between wet and dry as prevailing water stress, and the ability of the plant to change its development, as environmental conditions change, which is known as phenotypic plasticity. We already showed that some QTL are responsible for root plasticity, and root osmotic adjustment is one of the physiological bases for the plasticity. Then we have conducted a series of experiments to evaluate the functional roles of root plasticity by using various accessions/populations such as OryzaSNP germplasm set, chromosome segment substitution lines derived from Nipponbare and Kasalath cross, IR 64 NILs and a few promising genotypes including NERICA (New Rice for Africa that is an interspecific cross between Oryza sativa and Oryza glaberrima) lines. We have been using rootbox-pinboard method, slant tube method, experimental bed installed with line source sprinkler system that can create gradient in drought stress intensities, and sloping bed that can create increasing rooting depths under field conditions for phenotyping root traits. These results consistently showed that in addition of deep roots, the plastic development of root system was a key trait for plant adaptation to water stress and was expressed as an integration of rooting from tillers and their branching, with root plasticity triggered by water stress. We quantitatively showed the contributions of root plasticity to dry matter production through enhanced water uptake under water stress. Genotyping is now in progress by using some of the populations to determine the functions of genes responsible for the plasticity.