Background: Limonium bicolor, a halophytic species, can grow in saline or saline-alkali soil, is well known as a traditional Chinese medicine. Recently it attracted much attention for its treatment for cancer. Objective: The present study was performed to evaluate this species from the phytochemical standpoint and the possible relationship between the antitumor activity and its natural products. Materials and methods: The chemical constituents from the flowers of L. bicolor were investigated through bioassay-guided fractionation and isolation. All the individual compounds were characterized by spectroscopic analysis and their potential antitumor activity was tested against three different human tumor cell lines by MTT assays. Results: The EtOAc extract was proven as the most potent fraction and further fractionation led to the isolation of 15 natural flavonoids, which were characterized as luteolin (1), acacetin (2), quercetin (3), isorhamnetin (4), kaempferol (5), eriodictyol (6), kaempferol-3-O-α-L-rhamnoside (7), kaempferol-3-O-β-D-glucoside (8), quercetin-3-O-α-L-rhamnoside (9), quercetin-3-O-β-D-glucoside (10), quercetin-3-O-β-D-galactoside (11), myricetin-3-O-α-L-rhamnoside (12), kaempferol-3-O-(6″-O-galloyl)-β-D-glucoside (13), hesperidin (14) and rutin (15). The biotesting results demonstrated that both compounds 1 and 3 showed good cytotoxicity against human colon cancer cells (LOVO). Compound 5 exhibited relative greater growth inhibition against both human breast cancer cells (MCF-7) and osteosarcoma cell lines (U2-OS) at the concentration of 100 μg/mL. Conclusion: On the basis of these findings, the flavonoids were deduced to be potentially responsible for the antitumor activity of L. bicolor. The preliminary structure–activity relationship analysis suggests that the 3-O-glycosylation moiety in natural flavonoids was not essential for the antiproliferative activity on LOVO and U2-OS cells.