Petal flavonoid compositions of 39 tree peony cultivars from Xibei (northwest China) were investigated in order to study the chemotaxonomic relationship among tree peony species. Six anthocyanins, the 3- O-glucosides and 3,5-di- O-glucosides of three anthocyanidins-pelargonidin (Pg), cyanidin (Cy), and peonidin (Pn)-exist in petals without a blotch at the base. The flowers are classified into three anthocyanidin phenotypes: 'Pn, Pg>Cy'; 'Pn, Cy'; and 'Pn, Cy>Pg'. Furthermore, the yellow pigments are identified as three flavones and three flavonols: apigenin, luteolin, chrysoeriol, and kaempferol, quercetin, and isorhamnetin, respectively. Ward's minimum-variance cluster analysis with principal component analysis produced a dendrogram using standardized scores of 20 pigment variables. Of 39 cultivars, 11 clustered with white flowered Paeonia rockii and 17 with pink flowered P. rockii. The other 11 cultivars matched either P. delavayi or P. potaninii. The result suggests that the Xibei tree peony originated mainly from P. rockii.