Abstract: Effects of nitric oxide (NO) on ethylene biosynthesis in papaya fruit were investigated. Papaya fruit harvested at light green with some hint of yellow at the blossom end, were first fumigated with 60 μL/L NO for 3 h, then stored at 20℃ with 85% relative humidity (RH) for 20 days. Changes in the levels of ethylene, 1 -aminocyclopropane -1 -carboxylic acid (ACC), 1 - malonylaminocyclopropane-1-carboxylic acid (MACC), as well as the activities of ACC synthase (ACS), ACC oxidase (ACO) and the expression of CpACS2 and CpACO1 genes were measured during papaya ripening. The results showed that NO treatment inhibited ethylene production, ACO activity and the expression of CpACO1 gene, leading to the accumulation of ACC and MACC during storage.However, ACS activity and the expression of CpACS2 gene were not suppressed significantly by NO treatment.