Objective: To investigate the antiproliferative, anti-angiogenic, and apoptotic effects of extracts of Achillea biebersteinii (ABE) and combined treatments of ABE with 5-fluorouracil (5-FU) on HT-29 cells. Methods: The effects of ABE, 5-FU, and combined treatments on the viability of HT-29 cells were determined by 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Isobologram analysis was used to determine synergism between ABE and 5-FU. The apoptotic and anti-angiogenic effects were determined by cell death detection and human vascular endothelial growth factor ELISA method, respectively. Transcriptional and translational expressions of p53, Bax, Bcl-2, p38 MAPK, Akt, PTEN, and mTOR were also evaluated by real-time PCR and Western blotting analysis. Results: ABE decreased the viability of HT-29 cells in a dosedependent manner. Combined treatment of hexane, chloroform, and methanol extracts of Achillea biebersteinii with 5-FU at IC50 doses decreased the cell viability to 26.0%, 19.1%, and 14.9%, respectively (P<0.001). Furthermore, ABE treatment alone and combination with 5-FU, induced apoptosis, significantly downregulated mTOR, Akt, Bcl-2 expression, upregulated p53, Bax, PTEN, p38 MAPK expression, and exhibited anti-angiogenetic effects. Conclusions: Our findings indicate that ABE shows synergism with 5-FU and inhibits the proliferation of HT-29 cells by inducing apoptosis and suppressing angiogenesis, which may provide biological evidence for further use of ABE in the treatment of colorectal cancer.