N-Butylidenephthalide (BP), which is extracted from a traditional Chinese medicine, Radix Angelica Sinensis (danggui), displays antitumor activity against various cancer cell lines. The purpose of this study was to investigate the cytotoxic and radiosensitizing effect of BP and the underlying mechanism of action in human breast cancer cells. BP induces apoptosis in breast cancer cells, which was revealed by the TUNEL assay; the activation of caspase-9 and PARP was detected by western blot. In addition, BP-induced G2/M arrest was examined by flow cytometry and the expression levels of the G2/M regulatory protein were detected by western blot. BP also suppresses the migration and invasion of breast cancer cells, which was tested by wound healing and the matrigel invasion assay; the involvement of EMT-related gene expressions was detected by real-time PCR. Furthermore, BP enhanced the radiosensitivity of breast cancer cells, which was measured by the colony formation assay and comet assay, where the foci of γ-H2AX after radiation significantly increased in BP pretreated cells and was evidenced by immunocytochemistry staining and western blot. The homologous recombination (HR) repair protein Rad51 was down-regulated after BP pretreatment. These results indicate that BP might be a potential chemotherapeutic and radiosensitizing agent for breast cancer therapy.