© 2015, Science Society of Thailand under Royal Patronage. All rights reserved. The effects of salicylic acid (SA) on reactive oxygen species (ROS) and the antioxidant defence system of mango fruits cv. Nam Dok Mai No. 4 in cold storage were evaluated. Mature mangos were dipped in 1 mM SA and distilled water (as control) for 10 min and stored at 5±1°C with 90±2% RH for 42 days. Chilling injury (CI) and ROS levels including superoxide radical (O2^.-), hydrogen peroxide (H2O2) and hydroxyl radical (HO.) in the exocarp were measured every 7 days. The level of antioxidant system components, viz., enzymatic antioxidants: enzyme activity and gene expression of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX); and non-enzymatic antioxidants: ascorbic acid, total glutathione and total phenolic contents, total antioxidant capacity (TAC) were determined. In the control fruits, the levels of O2^.-, H2O2 and HO. increased during storage with a decrease in ascorbic acid and total glutathione contents, while total phenolic content, TAC, enzyme activities, and gene expression of SOD, CAT, and APX increased during the first 21 days of storage but thereafter gradually decreased. CI symptoms appeared in the control fruits on day 21 and rapidly increased thereafter. During storage, SA-treated mango fruits exhibited significantly higher activities of SOD, CAT, and APX; and increased levels of ascorbic acid, total glutathione, total phenolic compounds, and TAC compared to that of the control fruits. High activities of antioxidant enzymes were associated with higher gene expression, which was correlated with the reduction of CI in SA-treated fruits. We conclude that during cold storage, the SA-treatment strengthens the antioxidant defence system reducing CI in mango fruits cv. Nam Dok Mai No. 4 during cold storage.