Background: Toona sinensis (A. Juss.) Roemer is an endemic species of Toona genus native to Asia. Its crude extract exhibits an effective anti-oxidant capacity against oxidative models, but the intrinsic substances responsible for this capacity in the extract remains unclear and is yet to be studied comprehensively. Objective: To investigate the chemical constituents of the young leaves of Toona sinensis and its anti-oxidant capacity. Materials and Methods: Silica gel column chromatography, preparative high-performance liquid chromatography (HPLC), nuclear magnetic resonance (NMR), and mass spectrometry (MS) were used to isolate and characterize the chemical constituents. Four chemical-induced oxidative models including DPPH free-radical scavenging assay, phenazine methosulphate (PMS) nicotinamide adenine dinucleotide (NADH) PMS-NADH-NBT superoxide anion scavenging assay, FeCl 3 -K 3 Fe (CN) 6 reducing power assay, and FeCl 2 -FerroZine metal chelation assay were applied in the present study for evaluating anti-oxidant capacity. Results: Five flavonols and three derivatives of gallic acid, including quercetrin, kaempferol-3-O-α-L-rhamopyranoside, astragalin, quercetin, kaempferol, methyl gallate, ethyl gallate, and 1, 2, 3, 4, 6-penta-O-galloyl-β-D-glucopyranose were isolated from the leaves. Results showed that these compounds exhibited various antioxidant properties, markedly either as the strong scavengers for superoxide and free radicals or as molecules that were reducing or metal chelating in nature. Conclusion: The findings suggested that the 8 compounds in the young leaves of T. sinensis that were isolated in our study were the active compounds responsible for its antioxidant activity. These compounds can be utilized as a potential health supplement, as an available source of natural antioxidants, and as an effective material in pharmaceutical applications.