To study the chemical constituents of the flowers of Hylocereus undatus. The constituents were isolated and purified by various modern chromatographic techniques. The structures of the compounds were elucidated based on their physicochemical properties and spectral analyses. Thirteen compounds were isolated from the flowers of H. undatus. Their structures were elucidated as kaempferol (1), quercetin (2), isorhamnetin (3), kaempferol 3-O-alpha-L-arabinfuranoside (4), kaempferol 3-O-beta-D-glucopyranoside (5), quercetin 3-O-beta-D-glucopyranoside (6), isorhamnetin 3-O-beta-D-glucopyranoside (7), kaempferol 3-O-beta-D-galactopyranoside (8), quercetin 3-O-beta-D-galactopyranoside (9), kaempferol 3-O-beta-D-rutinoside (10), isorhamnetin 3-O-beta-D-rutinoside (11), kaempferol 3-O-alpha-L-rhamopyranosyl-(1 --> 6)-beta-D-galactopyranoside (12), and isorhamnetin 3-O-alpha-L-rhamopyranosyl-(1 --> 6)-beta-D-galactopyranoside (13). All the compounds are isolated from this plant for the first time.