Petai mistletoe (Dendrophthoe praelonga (Blume) Miq) is semi-parasitic plant. Parasite plant has been used traditionally in Indonesia to prevent or treat degenerative diseases, such as cancer. The purpose of this study was to indentify anticancer compound of petai mistletoe leaves extract and to examine its antioxidant activity (based on DPPH method) and cytotoxic activity on breast cancer (MCF-7) cell lines in vitro using Alamar blue method. This research aims to isolating, characterize, determine antioxidant activity and cytotoxic activity. Separation was conducting column chromathography and characterize was conducted using TLC, FT-IR, LC-MS and NMR-1D. From the result obtained, the isolated compound is flavonol quercetine (3,3,7,3’,4’ pethahydroxyflavone) which active antioxidant with IC50 value of 9.4 μg/mL and the value of cytotoxic activity with IC50 750.62 μg/mL