Tamoxifen is known as a standard therapeutic treatment for estrogen receptor-positive breast cancer, which down-regulates breast cancer mortality by 31% approximately. Carnosic acid is a phenolic diterpene, which has anti-cancer, anti-inflammation, anti-diabetic and anti-bacterial properties, generated by various species coming from Lamiaceae family. The breast cancer is reported as one of the most common tumors among women worldwide. In our study, the possible benefits of carnosic acid cooperation with tamoxifen for breast cancer treatment in vitro and in vivo were investigated. Carnosic acid and tamoxifen cooperation led to apoptosis in breast cancer cells. Caspase-3 signaling pathway was promoted for carnosic acid and tamoxifen co-treatment. Consistently, anti-apoptotic molecules Bcl-2 and Bcl-xl were down-regulated, while pro-apoptotic signals Bax and Bad were up-regulated. The elevation of decoy receptor 1 and 2 (DcR1 and DcR2) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) were enhanced for carnosic acid and tamoxifen cooperation. Furthermore, the mouse xenograft model in vivo suggested that carnosic acid and tamoxifen combined therapy inhibited breast cancer growth in comparison to the carnosic acid or tamoxifen monotherapy. Our study supplies a novel therapeutic strategy to induce apoptosis for suppressing breast cancer, which was relied on Caspase-3/TRAIL activation.