Abstract: Abstract: Cucumber (Cucumissativus L.) is an important economical crop, and its growth and yield are often dependent on the nitrogen (N) supply. Nitrogen deficiency can affect the development, quality and yield of cucumbers. Seeds of cucumber (C. sativus L. cv. Chinese long 9930) were surface-sterilized with 5% (w/v) NaClO for 15 min and germinated for 1 day at 30 ℃ in the dark. Germinated seeds were transferred to a greenhouse at the Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences (Beijing, China, 39.9°N), and uniform seedlings were transplanted into rectangular boxes (288 mm ×125 mm×75 mm) when the second leaf was expanding. All seedlings were grown in reformative Hoagland solution containing 12 mmol/L nitrogen. Equal numbers of seedlings were transplanted into new boxes containing reformative Hoagland solution with (12 mmol/L) or without (0 mmol/L) nitrogen when the seedlings had 5 leaves. Samples were collected at 0, 6, 12, 24, and 48 hours after the start of the treatment. The collected leaves were immediately frozen in liquid nitrogen and stored at -80 ℃. In this work, reactive oxygen species (ROS) and malondialdehyde (MDA) concentration and changes in enzymes involved in the antioxidant enzyme system (including SOD, CAT and POD) were investigated in nitrogen-deficient cucumber seedlings. The results showed that the N-deprivation increased ROS concentration in cucumber leaves significantly, consisted with the increasing MDA level. The content of H2O2 reached the peak at 6 hours of nitrogen deficiency (HND), which was about twice as much as the content of H2O2 at 0 HND (control). O2?- yield rate increased at around 37% at 6 HND compared with the control. The concentration of MDA increased the time and increased overtime by 25% approximately at 48 HND. If plants were harmed by oxidative stresses, much more ROS would be produced and the plants could not decompose them completely. In addition, MDA is the most abundant aldehydic lipid breakdown product in plants. Therefore, the increasing ROS and MDA concentrations showed that the cucumber seedlings were suffering with nitrogen stress. The antioxidant enzyme system was known as a system that can prevent plants from oxidative stresses. SOD, CAT and POD are the most important enzymes of this system. The response of these enzymes is helpful for plants to resist stresses. SOD can transformed the O2?- into H2O2, and then the H2O2 can change to H2O by CAT and POD. In this study, the activities of SOD, CAT, and POD increased significantly under nitrogen deficiency. For N-deficient cucumber leaves collected at 48 HND, the activities of SOD, POD CAT and increased by about 37%, 24% and 26%, respectively. The changes of transcript levels of SODs, CATs and PODs were consistent with the changes of those activities. SOD2 transcript level was significantly increased at 48 HND, which was about 6 times as much as the transcript level of SOD2 at 0 HND. The transcript levels of POD1 and CATs increased 10 times approximately after the treatment. These results indicated that nitrogen deficiency induced the anti-oxidase defense system in cucumber leaves.