Previous studies have shown that modification of activating transcription factor 6 (ATF6) protein is important for the endoplasmic reticulum (ER) stress response; ER stressors stimulate the degradation of ATF6 by Site-1 protease (S1P) and Site-2 protease (S2P) into p50-ATF6, which acts as a transcription factor. In the current study, we found that all of the ER stressors tested (such as thapsigargin) up-regulate ATF6 mRNA expression. As thapsigargin did not affect the stability of the ATF6 mRNA, it was concluded that this up-regulation is due to transcriptional activation of ATF6. An inhibitor of S1P suppressed this up-regulation of ATF6 mRNA expression and putative ATF6-binding elements in the promoter of ATF6 were identified, suggesting that p50-ATF6 positively regulates the gene expression of ATF6. Since cells over-expressing ATF6 showed an enhanced ER stress response, we propose that up-regulation of ATF6 mRNA expression is involved in enhancing the ER stress response.