Abstract: To analyze the adaptation of Hippophae rhamnoides L. to water condition of the semiarid loess hilly region, water physiological and ecological characteristics of Hippophae rhamnoides L. and drought resistance were studied in 1998. The following results are obtained: (1) Hippophae rhamnoides L. forms the way of drought resistance of dry hardiness and keeping out drought to adapt the habitat of semiarid loess hilly region. In dry season, Hippophae rhamnoides L. shows stronger drought resistance, leaves of Hippophae rhamnoides L. can keep higher relative water content, weak transpiration intensity, low leaf water potential, big critical saturation deficiency, higher bound water content and bigger ratio of bound water to free water. (2) Photosynthetic rate of Hippophae rhamnoides L. has obviously diurnal variation,first peak assumed at 10:00 12:00, the second at 14:00 15:00. Photosynthetic rate has significant correlation with environmental factors (temperature, relative humidity and photosynthetic active radiation), and the coefficient was 0.846 6 0.935 1, of which photosynthetic active radiation has the biggest effect on photosynthetic rate, followed by temperature and relative humidity. Photosynthetic rate has utterly significant correlation with stomatal conductance and intercellular CO2 concentration, and the coefficient is 0.985 2 0.985 6.(3)Transpiration rate and stomatal conductance have a similar diurnal variation, both exhibiting a single peak pattern in May and July, and bimodal pattern in June? August and September. In growing season, May through September, the maximum of transpiration occurs in July, lower value in May and September; Transpiration rate in May (0.39 g·g-1 ·h -1) was just 41.93% of that in July (0.93g·g-1 ·h-1). The result indicates that precipitation primarily could meet the need of growth of Hippophae rhamnoides L. to the environmental condition of Ansai loess hilly region, Hippophae rhamnoides L. can be selected as pioneer species for planting in this region. These provide scientific basic for drought resistant forestation of Hippophae rhamnoides L., introducing fine variety, breeding and improving management.