The effects of three nitrate levels - 14(CK), 56(T-1), and 140 mmol L-1 (T-2) - on growth and photosynthetic capacity of cucumber (Cucumis sativus L. cv. Xintaimici) seedlings grown in hydroponic culture were investigated. The results showed that at 12 d after treatment plant height, stem diameter, leaf area, and leaf number of cucumber seedlings were stimulated by 56 mmol L-1 nitrate, whereas were inhibited significantly by 140 mmol L-1 nitrate compared with CK. Short-term stimulation in photosynthetic rate occurred under T-1 treatment, and then recovered to the level of CK. Photosynthetic rate of T-2 seedlings significantly decreased over treatment course with respect to CK. Photosynthetic pigment content of T-1 and T-2 increased during the first 2 d, and gradually recovered to the level of CK thereafter. Chlorophyll a/b and carotenoids/chlorophyll of T-1 had no significant difference from CK during treatment period. During the first 4 d, there was no significant difference in chlorophyll a/b and carotenoids/chlorophyll between T-2 and CK. After 4 d, chlorophyll a/b of T-2 increased gradually, whereas carotenoids/chlorophyll decreased. Actual PSII efficiency (ΦPSII) and photochemical quenching (qP) of T-1 had no significant difference from CK, and non-photochemical quenching (qN) was a little higher than CK after 2 d. During the first 2 d, there was little difference in ΦPSII and qP between T-2 and CK. After 2 d, both ΦPSII and qP of T-2 decreased to a great extend. A significant increase in qN of T-2 occurred over treatment course. With respect to CK, Hill reaction activity of T-1 slightly decreased, and T-2 treatment resulted in a significant decrease of Hill reaction activity. This evidence indicates that high-level nitrate stress may reduce photosynthesis through its effects not only on stomatal conductance but on the photosynthetic apparatus.