Background: The screening of Indonesian medicinal plant extracts against T47D cell line and identifying active chemical constituents of Alpinia galanga rhizome were conducted. Materials and Methods: Thirty methanol extracts of Indonesian medicinal plants were screened for possible anti-breast cancer. The T47D cell line was used as a target model. Chromatography techniques were used to purify the chemical constituents of A. galanga rhizome. Structural elucidation of isolated compounds was conducted by means of 1D and 2 D NMR (nuclear magnetic resonance) spectrometry. Their activities were also examined on MCF7, WiDr, and HeLa cell lines. Results: Five samples (A. galanga, Clonorchis sinensis, Piper cubeba, Santalum album, and Vitex trifolia) showed a strong activity with 96.4%, 91.9%, 87.6%, 82.6%, and 88.7% inhibition, respectively. Since A. galanga exhibited the most potent activity, its IC50value was determined with a dose-dependent effect with the IC50value of 21.2 μg/mL. Its methanol extract was also separated based on chromatography techniques producing the following four compounds: trans-p-coumaryl alcohol (1); p-coumaryl acetate (2); [1'S]-1'-acetoxy chavicol (3); and trans-p-coumaryl diacetate (4). Compounds 3 and 4 showed significant activity with the IC50values of 17.3 and 25.4 μg/mL, respectively. On the other hand, compounds 1 and 2 showed weak activity. All compounds exhibited a similar feature against MCF7, WiDr, and HeLa cell lines. Conclusions: On the structure-activity relationship observation, acetoxy group at a para position could be a key contributor to the effect. Thus, an acetoxy phenylpropanoid could be a good model for future anti-breast cancer lead compound development. Furthermore, extract plants should have demonstrated their potential to inhibit cancer cells.