Abstract: Nitrate(NN) and ammonium(AN) were primary nitrogen forms of root uptake in high plant. It has been found that nitrate promoted plant growth, while high-level AN restrained it. The effects of nitrogen form and level on dry matter accumulation, turgor pressure and photosynthesis of hydroponically cultivated cucumber (Cucumis sativus) seedlings were investigated. The results showed that nitrate treatment could increased the shoot dry matter, leaf area, leaf water content, water potential turgor pressure and photosynthetic rate of cucumber seedlings compared with AN treatment. Though low-level AN (5mmol/L) supply may affect leaf development of cucumber seedlings, it had no negative effect on photosynthetic rate. Compared to the same level of NN, low-level AN had higher photosynthetic rate. While under high-level AN, the dry matter accumulation of cucumber seedlings was significantly restrained, minimum Pn was CO2 1.6 mol/ (m2·s), and leaf area and Pn was only 12.7% and 10% of them at the same level of NN, respectively.. AN supply may cause leaf area decrease, which was more dependent on the reducing of its leaf turgor pressure, and relative coefficient of them was 0.98; the fall of turgor pressure was mainly correlated with water potential, but osmotic potential. Cucumber seedlings accumulated abundant osmotic adjustment and were osmotic stress under high nitrate, so the growth was restrained. The growing status of Xintaimici (XTMC) was better than Jiniluo (JNL) at all treatments. Low AN supply had little negative effect on dry matter accumulation; while high-level AN significantly inhibited cucumber growth. So we can concluded that high-level AN brought toxicity to plant, while high-level NN brought osmotic stress to it.