Proteasome inhibitors represent a novel class of anti-tumor agents with pre-clinical and clinical evidence of activity against hematologic malignancies and solid tumors. Emerging lines of evidence suggest that the unfolded protein response (UPR) is implicated in proteasome inhibitors-induced apoptosis. Glucose-regulated protein 78kD (GRP78) and CHOP as part of the UPR, play critical roles in cell survival or death. Here we demonstrate that induction of GRP78 and CHOP are differently regulated upon proteasome inhibition in different thyroid cancer cell lines and GRP78 levels as well as preferential induction of GRP78 or CHOP appears to be involved in the responsiveness. Insensitive ARO, 8305C and 8505C cell lines inherently express relatively high levels of GRP78 compared to sensitive cell lines, and its levels are further upregulated upon treatment with proteasome inhibitors. CHOP levels are dramatically induced in sensitive cell lines till 24 hours after proteasome inhibition. On the other hand, only a slight increase is observed at 4 hours in insensitive cell lines and this increase is unable to be detected after 8 hours. Insensitive cells are sensitized to proteasome inhibition by suppression of GRP78. Furthermore, suppression of CHOP induction or overexpression of GRP78 partially prevents proteasome inhibition-mediated cell death. Our study indicates a molecular mechanism by which the sensitivity of thyroid cancer cells is regulated by the level of GRP78, as well as preferential induction of GRP78 or CHOP upon treatment with proteasome inhibitors. Our experiments therefore suggest a novel approach towards sensitization of thyroid cancer cells to proteasome inhibitors.