Cucumber cultivar ‘Jinyou 4’ was subjected to low temperature treatment of 18 ℃/10 ℃(day/night) in the artificial climate chamber.The effects of exogenous melatonin(MT) on the growth and antioxidant system of leaves in cucumber under low temperature and weak light stress were investigated.The results showed that:compared with the control treatment,exogenous MT treatment could significantly promote the increment of plant height,wall thickness,flesh weight,dry weight;the chlorophyll content and root activity increased by 13.3% and 18.8%,GSH content and ASA content by 39.3% and 24.7%,respectively and increased SOD,POD,CAT,APX,membrane H⁺-ATP activities,while reduced MDA content and electrolyte leakage by 28.7% and 29.7% in cucumber seedlings leaves under low temperature and weak light stress.So we can conclude that exogenous MT treatment could raise the activities of antioxidant enzymes and matter,membrane H⁺-ATP activities,decrease the membrane lipid peroxidation,maintain membrane integrity and function,and improve the adaptability of cucumber seedlings under low temperature and weak light stress,the 200 μmol·L^-1 MT treatment was better.