The fruit of Citrus unshiu MARKOVICH used for various purposes in traditional medicine has various pharmacological properties including antioxidant, anti-inflammatory, and antibacterial effects. Recently, the possibility of anti-cancer activity of the extracts or components of this fruit has been reported; however, the exact mechanism has not yet been fully understood. In this study, we evaluated the anti-proliferative effect of water extract of C. unshiu peel (WECU) on human breast cancer MCF-7 cells and investigated the underlying mechanism. Our results showed that reduction of MCF-7 cell survival by WECU was associated with the induction of apoptosis. WECU-induced apoptotic cell death was related to the activation of caspase-8 and -9, representative initiate caspases of extrinsic and intrinsic apoptosis pathways, respectively, and increase in the Bax : Bcl-2 ratio accompanied by cleavage of poly(ADP-ribose) polymerase (PARP). WECU also increased the mitochondrial dysfunction and cytosolic release of cytochrome c . In addition, AMP-activated protein kinase (AMPK) and its downstream target molecule, acetyl-CoA carboxylase, were activated in a concentration-dependent manner in WECU-treated cells. In contrast, compound C, an AMPK inhibitor, significantly inhibited WECU-induced apoptosis, while inhibiting increased expression of Bax and decreased expression of Bcl-2 by WECU and inhibition of WECU-induced PARP degradation. Furthermore, WECU provoked the production of reactive oxygen species (ROS); however, the activation of AMKP and apoptosis by WECU were prevented, when the ROS production was blocked by antioxidant N -acetyl cysteine. Therefore, our data indicate that WECU suppresses MCF-7 cell proliferation by activating the intrinsic and extrinsic apoptosis pathways through ROS-dependent AMPK pathway activation.