In this study, we found that sirtinol (SIRT1 inhibitor) could block the inhibitory effect of losartan on the UUO-induced renal fibrosis, suggesting that the anti-fibrotic function of losartan might be mediated at least in part through the inhibition of ER stress via the up-regulation of SIRT1. Here, SIRT1 is linked to renal fibrosis.