Tamoxifen is an effective treatment for estrogen receptor alpha positive (ERα+)-breast cancer, however patients who received the treatment for five years have greater mortality risk compared to those who did not receive tamoxifen. Furthermore, patients treated with tamoxifen developed resistance to the drug which mediated through p53 and PTEN. Therefore, the study is undertaken to determine the potential adjuvant properties of flavonoids, apigenin and rutin to promote the anticancer activity induced by tamoxifen using ERα+-breast cancer MCF-7 cell lines. MCF-7 and non-transformed breast MCF-10A cells were treated separately with apigenin, rutin, tamoxifen or the combination of each flavonoids with tamoxifen. Anti-proliferative activity and respective IC50 concentrations were determined using MTT assay. The respective IC50 concentrations obtained were used in the subsequent experiments. The anti-proliferative mechanism was determined using Annexin V-FITC morphological staining and DNA fragmentation assays. The effect on tumor suppressor (p53 and PTEN) and cell cycle related genes (p21, CDK1 and Cyclin B1) were determined by QuantiGene Plex assay. Our results showed that MCF-7 cells were more sensitive to both apigenin and rutin compared to MCF-10A cells. Both cells were sensitive to tamoxifen. Apigenin and rutin synergistically enhanced tamoxifen anti-proliferative effect in MCF-7. Meanwhile rutin protects MCF-10A against the toxicity of tamoxifen. Our results indicate that the anti-proliferative mechanism of apigenin and rutin is mediated by apoptosis signals. In MCF-7 cells, both tumor suppressor (p53 and PTEN) and cell cycle related genes (p21 and CDK1) were up regulated by apigenin and rutin, contrary to tamoxifen. Apigenin and rutin induced G2/M arrest and apoptosis in MCF-7 cells through p53-dependent pathway. Both flavonoids are suggested as potential adjuvant agents to enhance tamoxifen efficacy in ERα+-breast cancer treatment. © 2018 Penerbit UTM Press. All rights reserved.