Abstract: Effects of supplementary illumination 0, 3, 6, 9 and 12 h at night on growth, endogenous hormones content and nutrient absorption of cucumber (Cucumis sativus L.) seedlings (‘Jinyan 4’) were studied to probe the role and mechanism of supplementary illumination in breeding seedlings. The results showed that: compared with the control (supplementary illumination 0 h) dry weight, leaf area, R/S, seedling index, mineral elements accumulation, indole acetic acid (IAA), gibberellin (GA3), (ZR+GA3+IAA)/ABA and root activity increased under supplementary illumination, while abscisic acid (ABA) decreased, and the changes of these indicators were more pronounced as the supplementary illumination time prolonging. Zeatin riboside (ZR) content firstly increased, then decreased with the supplementary illumination time prolonging, and it is the highest when supplementary illumination time was 6 h. These results suggested that supplementary illumination at night can promote the growth of cucumber seedlings, and that was connect with the changes of endogenous hormone content, root activity and accumulation of mineral elements.