Phytochemical study on the EtOAc fraction from a MeOH extract of the leaves of Alnus hirsuta Rupr. led to the isolation of nine compounds betulin (1), betulinic acid (2), hirsutanonol (3), hirsutenone (4), quercetin (5), avicularin (6), gallic acid (7), hyperin (8), and daucosterol (9). Among them, six compounds 1, 2, 57, and 9 are report from this plant for the first time. All isolated compounds were evaluated for their antioxidant activity using DPPH radical scavenging capacity and inhibition effect on mitochondrial lipid peroxidation. Six phenolic compounds 3-8 were found to have potent antioxidant activity. Of which, compounds 3, 4 and 5 showed significant free radical scavenging activity with the IC50 values of 18.3 ± 2.5, 15.7 ± 3.8 and 23.5 ± 3.1 μm, respectively. In addition, the compounds 3-8 exhibited inhibition effect on the mitochondrial lipid peroxidation with the IC50 values of 88.0 ± 6.5, 12.6 ± 1.2, 8.0 ± 1.1, 58.5 ± 4.3, 173.6 ± 15.2, and 75.0 ± 6.7 μm, respectively.