Mitochondria and the Nox family of NADPH oxidase are the two major sources of reactive oxygen species (ROS) that are induced by external stimuli, and the mitochondria respiratory chain is considered as an important site of ROS production within most cells. Recent evidence has demonstrated that various biological stimuli increase accumulation of unfolded or mis-folded proteins in ER lumen, which is referred to as "ER stress". Moreover, these various pathologic stimuli have been reported to provoke oxidative stress as well as ER stress. In this study, we used the mice sensitized with OVA and LPS and then challenged with OVA (OVALPS-OVA mice) for elucidation of the relationship between mitochondrial ROS and ER stress in bronchial asthma. The OVALPS-OVA mice showed that the expression of ER stress markers and the protein levels of un-folded-protein response (UPR)-related marker in lung tissues were significantly increased after OVA challenge. In addition, we visualized the localization of mitochondrial ROS in BAL cells isolated from OVALPS-OVA mice using confocal microscopy; the significant increase in mitochondrial ROS in BAL cells was observed after OVA challenge. Our results also showed that Necrox-5 or 4-PBA significantly reduced the increases in ER stress, mitochondrial ROS, inflammatory cytokines, airway in-flammation, and bronchial hyperresponsiveness. These findings suggest that mitochondrial ROS and ER stress plays an im-portant role in the induction and maintaining allergic airway diseases synergistically.