Abstract: Antioxidant activity and structure-activity relationship of ethanol extract from pine needle of Cedrus deodara were investigated. The antioxidant activity of ethanol extract from pine needle was determined by some chemical analytical methods in vitro. Folin-ciocalteu colorimetry and aluminum trichloride colorimetry were used to determine the total phenols content and total flavonoids content,respectively. HPLC-MS and NMR were used to identify the main compounds and the structure-activity relationship was also discussed initially. The results indicated that the pine needle ethanol extract possessed the markable ability to scavenge ABTS(EC₅₀=0.48±0.01 mg/mL)and DPPH(EC₅₀=32.40±0.76 μg/mL)free radicals. Reactive oxygen from hydroxyl radical(EC₅₀=1.73±0.01 mg/mL)and hydrogen peroxide(EC₅₀=89.48±0.54 μg/mL)were scavenged by the extract significantly. Efficient ability of pine needle ethanol extract to inhibit the lipid peroxidation(EC₅₀=9.85±0.14 mg/mL)and reduce ferric ion were also observed. The pine needle ethanol extract had high total phenol(40.30±2.13 mg gallic acid equivalent/g extract)and total flavonoids(56.62±1.53 mg rutin equivalent/g extract)contents. Methylconiferin,ferulic acid-β-D-glucoside,wikstromol,secoxyloganin,(E)-1-O-p-coumaroyl-α-D-glucopyranoside and pelargonidin-3-O-glucoside were identified as the main compounds,and their phenolic hydroxyl,methoxy and benzyl groups might be responded for the remarkable antioxidant activity.