Heat shock protein 70 (HSP70) is frequently overexpressed in a variety of human malignancies and protects cancer cells against apoptosis in response to various stresses. The bioflavonoid quercetin inhibits HSP70 expression and induces cancer cells apoptosis. In the present study, we have investigated the effects of HSP70 down-regulation on the unfolded protein response (UPR) and addressed a novel strategy to enhance the proapoptotic effect of quercetin by suppressing GRP78 induction simultaneously. Treatment of human breast cancer cells with quercetin down-regulates HSP70 expression, but up-regulates GRP78 expression in a dose-dependent manner. Down-regulation of HSP70 by small interfering RNA also leads to induction of GRP78. Moreover, our studies reveal that HSP70 knockdown or quercetin induces other typical components of the UPR, including CHOP expression, eIF2alpha and JNK phosphorylation, caspases activation and XBP-1 splicing. Abrogating the induction of pro-survival chaperone GRP78 by small interfering RNA sensitizes breast cancer cells to quercetin. Colony survival assays demonstrate that treatment of breast cancer cells with green tea (-)-epigallocatechin gallate (EGCG), which binds to the ATP-binding domain of GRP78 and blocks its protective function, synergistically promoted quercetin-induced cell death. These studies reveal that HSP70 down-regulation leads to the induction of UPR. The pro-survival GRP78 induction contributes to quercetin resistance. Abrogation of GRP78 induction or inhibition of GRP78 activity increases the effectiveness of quercetin. These findings indicate that combinational administration of flavonoids capable of suppressing HSP70 and GRP78 such as quercetin and EGCG might represent a novel approach for cancer therapy or chemoprevention.