The controlled and moderate oxidative stress such as ozone induces both inflammatory and anti-inflammatory response. This balance is important for homeostasis of living organisms. Furthermore, it has been shown that this conflict response is mainly regulated by two transcriptional factors, nuclear transcriptional factor κB (NF-κB) and nuclear factor erythroid 2-related factor 2 (Nrf2). NF-κB is involved in inflammatory responses by regulating expression of cyclooxygenase-2 (COX-2) and various inflammatory cytokines while Nrf2 is involved in anti-inflammatory responses by controlling expression of numerous antioxidant enzymes such as heme oxygenase- 1 (HO-1). We here demonstrate the molecular mechanisms of the crosstalk between NF-κB and Nrf2 activation during the moderate oxidative stress induced by ozone. We first confirmed the activation of NF-κB and Nrf2 signaling during the moderate oxidative stress in HeLa cells. Induction of NF-κB-mediated COX-2 mRNA expression was observed at the early phase after stimulation (30-60 min after ozone treatment). However, induction of HO-1 mRNA expression was observed at the late phase of stimulation (6 h after stimulation). To reveal the crosstalk between NF-κB and Nrf2, we tested whether reduction of NF-κB expression affects ozone-induced Nrf2 activation by knocking down of NF-κB in HeLa cells. Importantly, the HO-1 induction by ozone was remarkably decreased by a reduction in NF-κB expression. These results suggest that the moderate oxidative stress by ozone initially induces NF-κB activation, and this NF-κB activation is required for HO-1 induction at the late phase of the moderate stress.