Diallyl disulfide (DADS) is one of the major volatile components of garlic oil. DADS has various biological properties, including anticancer, antiangiogenic, and antioxidant effects. However, the anticancer mechanisms of DADS in human breast cancer have not been elucidated, particularly in vivo. In this study, we demonstrated that the expression of miR-34a was up-regulated in DADS-treated MDA-MB-231 cells. miR-34a not only inhibited breast cancer growth but also enhanced the antitumor effect of DADS, both in vitro and in vivo. Furthermore, Src was identified as a target of miR-34a, with miR-34a inhibiting SRC expression and consequently triggering the suppression of the SRC/Ras/ERK pathway. These results suggest that DADS could be a promising anticancer agent for breast cancer. miR-34a may also demonstrate a potential gene therapy agent that could enhance the antitumor effects of DADS.