Growing interest in the beneficial effects of antioxidants has inspired the synthesis of new phenolic acid phenethyl ureas (PAPUs) with enhanced antioxidant potential. We have previously shown the capacity of one PAPU compound, (E)-1-(3,4-dihydroxyphenethyl)-3-styrylurea (PAPU1), to induce caspase-dependent apoptosis in melanoma cells. In t he p resent s tudy, we exam ined t he a nti-proliferative effects of PAPU compounds on MCF-7 human breast cancer cells and determined the molecular mechanisms involved. Treatment with PAPU compounds inhibited predominantly proliferation in these cells, where the PAPU1 was the most efficient form. Flow cytometric analysis showed that PAPU1 blocked cell cycle progression in the $G_0/G_1$ phase, and reduced the proportion of cells in $G_2$/M phase. This was related to the inhibition of cell cycle regulatory factors, including cyclin D/E and cyclin-dependent kinase (CDK) 2/4, through induction of $p21^{Cip1}$. PAPU1 also induced the mitochondrial-mediated and caspase-dependent apoptosis in MCF-7 cells. This was evidenced by cellular changes in the levels of Bcl-2 and Bax, loss of the mitochondrial membrane potential, release of cytochrome c into the cytosol, and caspase-9 activation. Collectively, our results suggest that $G_1$ cell cycle regulatory proteins and mitochondrial pathways are the crucial targets of PAPU1 in the chemoprevention of breast cancer cells.