The Bax inhibitor-1 (BI-1) is an anti-apoptotic protein that is located in ER membranes and protects cells from ER stress-induced apoptosis. The ER is associated with generation of reactive oxygen species (ROS) through oxidative protein folding. This study examined the role of BI-1 in the regulation of ER stress-induced accumulation of ROS and expression of unfolded protein response (UPR)-associated proteins. BI-1 reduced the expression levels of GRP78, CHOP, phospho-eIF-2a, IRE1a, XBP-1 and phospho-JNK and inhibited the cleavage of ATF-6 p-90, leading to the inhibition of ROS. Although ROS scavengers offer some protection against ER stress-induced apoptosis, the expression of pro-apoptotic ER stress proteins was not affected. This study shows that the response of unfolded proteins is followed by ROS accumulation under ER stress, which is regulated in BI-1 cells. The mechanism for these BI-1-associated functions involves the expression of heme oxygenase-1 (HO-1) through Nrf-2. In BI-1 cells, the transfection of HO-1 siRNA completely abolished the BI-1-induced protection. Treatment of the cells with ZnPP, a HO-1 inhibitor, had a similar effect on the BI-1 protection model to that observed with HO-1 siRNA. Furthermore, the C-terminal-deleted BI-1-overexpressed cells reversed the BI-1-induced protection with the abrogation of HO-1 induction. The endogenous expression of HO-1 through ER stress-initiated ROS is believed to be as a protection signal. In conclusion, these observations suggest that BI-1 can inhibit the ER stress proteins as well as the accumulation of ROS thereby protecting the cells. Moreover, HO-1 plays an important role in the BI-1-associated protection against ER stress.