Abstract: Root morphological characteristics are important parameters for evaluating plant adaptability to environmental stress. To clarify the relationship between the root morphological characteristics of old world bluestem (Bothroichloa ischaemum) and soil water conditions, pot experiments were conducted to investigate root biomass, total root length, root surface area, average root diameter, specific root length and specific root area under three soil water regimes: (1) sufficient water supply (HW, 80% field capacity), (2) moderate water stress (MW, 60% field capacity), and (3) severe water stress (LW, 40% field capacity). Moreover, soil water contents were improved from MW to HW, LW to HW and LW to MW at the three main growth periods (jointing, flowering and maturing). At the end of the growth period, root biomass and morphological characteristics under the different water treatments were measured and analyzed. The results showed that old world bluestem root growth was closely related to soil water conditions. Root biomass, total root length and root surface area were also significantly higher with continuous sufficient water supply than under the other treatments (P<0.05). Water stress inhibited the root growth of old world bluestem but there were no significant differences in average root diameter between the different water treatments. Positive linear relationships were found between root biomass and total root length as well as with root surface area. The roots exhibited strong morphological plasticity in response to changes in soil water conditions. Higher levels of water content improvement significantly improved root biomass, total root length and root surface area, especially during the jointing period. These results suggest that old world bluestem has strong water stress tolerance and that the growth response of its roots to soil water change is related to the plant growth period as well as to the degree of water stress prior to improvements in soil water content.