Chemical constituents of the bark of Swietenia macrophylla King (Meliaceae) was investigated not only to develop further bark utilization but also to understand the biochemical function of the bark in the forest environment. A new phenylpropanoid-substituted catechin, namely, swietemacrophyllanin [(2R*,3S*,7"R*)-catechin-8,7"-7,2"-epoxy-(methyl 4",5"-dihydroxyphenylpropanoate)] (1) was isolated from the bark of S. macrophylla together with two known compounds, catechin (2) and epicatechin (3). The structure of 1 was elucidated by spectroscopic data and by comparison of the NMR data with those of catiguanins A and B, phenylpropanoid-substituted epicatechins. The 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity of the isolated compounds indicated that all of the three compounds have strong activity compared with trolox as a reference. Swietemacrophyllanin (1) had the strongest activity with a 50% inhibitory concentration (IC50) value of 56 microg mL(-1).