The objectives of this study were to understand the tolerance mechanism of woody plants to water stress and tolerance changes in relation to mycorrhizal formation. Lespedeza cyrtobotrya Miq. commonly used for erosion control in slopes were raised from seeds and transplanted to 120 plastic pots. Sixty pots received the top soil of a Fraxinus americana forest, while remaining 60 pots received the autoclaved top soil. The forest soil contained 1,200 spores per 100g of arbuscular endomycorrhizal fungus, mostly Glomus sp. The plants were raised outside with regular supply of water and mineral nutrients. Two kinds of water deficit treatment and a control were started at the middle of July : cyclic water deficit treatment with 3 cycles of sequential water stress at the point of xylem water potential of about -0.6, -0.6, and -1.7 MPa and recovery, and non-cyclic water deficit treatment with single water stress at about -1.5 MPa. The non-stressed plants received plenty of water throughout the period. In late August the plants were harvested for measurements of dry weight, N, P, carbohydrate contents, net photosynthesis and superoxide dismutase(SOD) activities. Both cyclic and non-cyclic water deficit treatments reduced dry weight by 60% and 40%, respectively, and reduced nitrogen absorption, while increased SOD activities. Water-stressed plants also showed increased carbohydrate contents in the leaves and lowered stomatal conductance. Mycorrhizal inoculation resulted in an average of 40% infection of roots and 2-3 times increase in P absorption in water-stressed as well as non-stressed plants. Mycorrhizal formation also increased shoot-root ratio. The results that SOD activities of water-stressed plants with mycorrhizal infection were significantly lower than those of non-mycorrhizal plants suggest the possibility of improvement of water-stressed condition by mycorrhizal formation. It was concluded that endomycorrhizal formation increased tolerance of Lespedeza cyrtobotrya seedlings to water stress.