Background: The purpose of this study was to investigate the phenolic composition and antioxidant activity of Thymbra sintenisii subsp. isaurica extract (TSIE) and, to evaluate, for the first time, anticancer effect on human MCF-7 (breast carcinoma) cells. Materials and Methods: The antioxidant capacity of TSIE was evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and total polyphenol assays. The anticancer activities of TSIE were tested on MCF-7 (breast carcinoma) cells. Results: Total polyphenol value of extracts TSIE was found as 73.02 mg gallic acid /g powder. DPPH result of IC50 value of TSIE was found to be 27.15 mu g/mL. To examine anticancer effect of TSIE at different concentrations were given on MCF-7 cells. TSIE was observed to reduce the cell viability in a dose-dependent manner. This anticancer property of the TSIE provides a highlights the importance of plant research for drug design. Conclusion: In this study, anticancer effects and antioxidant level of endemic species, that is TSIE, are evaluated on MCF-7 cells. Thus, an effective therapeutic agent for cancer treatment is aimed to develop. Further studies are needed to better understand the molecular mechanisms underlying this effect of TSIE.