Phytochemical study on the EtOAc fraction from the MeOH extract of the leaves of Cedrela sinensis led to the isolation of five known phenolic compounds (1-5), whose structures were identified as (+)-catechin (1), $kaempferol-3-0-{alpha}- L-rhamnopyranoside$ (2), quercetin (3), $quercetin-3-O-{alpha}-L-rhamnopyranoside$ (4), and $quercetin-3-O-{eta}-D-glucopyranoside$ (5), respectively, by comparing their spectral $(uv,;JR,;IH;and;^{13}C-NMR,;and;ESI-MS)$ and physicochemical data with those reported in the literature. Among the isolated compounds (1-5), compounds 1 and 3-5 exhibited significant DPPH radical scavenging effects with $IC{_50}$ values ranging from $21.3{pm}1.4;to;38.1{pm}3.2 {mu}M$ as well as superoxide anion radical scavenging effects with $IC{_50}$ values ranging from $9.4{pm}0.7;to;21.2{pm}3.6 {mu}M$. Furthermore, compounds 1 and 3-5 also exhibited considerable inhibitory effects on LDL peroxidation induced by either $CU^{2+}$ or AAPH with $IC{_50}$ values ranging from $1.4{pm}0.4;to;11.9{pm}1.4;{mu}M$. These results indicated that flavonoids are the major constituents of C. sinensis and considered to be antioxidant principles of this plant.