Wen-Pi-Tang, a traditional Chinese prescription, has been widely used for the treatment of patients with moderate chronic renal failure in China. Although the protective effect of Wen-Pi-Tang on peroxynitrite (ONOO(-))-induced renal tubular epithelial LLC-PK(1) cell damage was elucidated in our previous research, the active components of Wen-Pi-Tang have not yet been fully clarified. Therefore in the present study, we investigated the active components by using a cellular ONOO(-)generation system. As a result, p-coumaric acid, 4-(4'-hydroxylphenyl)-2-butanone 4'-O-glucopyranoside, gallic acid 3-O-(6'-O-galloyl)-beta-d-glucopyranoside, procyanidin B-1, procyanidin B-3, and (+)-catechin were isolated as active compounds inhibiting cellular ONOO(-) formation and cytotoxicity. In particular, the content of (+)-catechin was significantly higher than those of the other compounds, and the (+)-catechin structure was located in procyanidins B-1 and B-3. Therefore, the major bioactivity of Wen-Pi-Tang against ONOO(-)-induced cytotoxicity in LLC-PK(1) cells was thought to be mediated by (+)-catechin. Although we cannot disregard the synergetic effect of various components in Wen-Pi-Tang, (+)-catechin is a major active compound protecting against ONOO(-)-induced LLC-PK(1) cell damage and may be used as an index to qualify the ONOO(-)-inhibitory activity of Wen-Pi-Tang extract.