Objective　To investigate the functions and mechanisms of transcriptional regulation changes in estrogen receptor (ER) on letrozole resistance in breast cancer cells. Methods　Letrozole resistant cells (MCF-7-LR-1 and MCF-7¬LR-2) and control cells (MCF-7-Aro and MCF-7-T) were used in present study. The levels of ER phosphorylation at Ser-118 and expressions of ER in these two groups were determined by Western blotting. The conjugation of ER with XBP-1 enhancer and TFF-1 promoter with or without β-Estradiol (E2) was detected by Chromatin Immuno-coprecipitation (ChIP) assay. The mRNA expression levels of XBP-1 and TFF-1 were determined by quantitative reverse transcription PCR (qRT-PCR) analysis. Furthermore, the mRNA expressions of estrogen responsive genes (ERGs) including CA2, GJA1, PGR, and CTSD were assayed also by qRT-PCR analysis. Results　The level of ER phosphorylation at Ser-118 significantly increased in MCF-7-LR cells than in control cells. However, the ER expression showed no significant change. ChIP assay demonstrated that, under ligand-independent (without E2 treatment) or ligand-dependent (with E2 treatment 10nmol/L E2 treatment for 45min) condition, the conjugation of ER with XBP¬1 enhancer and TFF-1 promoter significantly increased in MCF-7-LR-2 cells than in control cells. qRT-PCR analysis showed that the mRNA expressions of XBP-1 and TFF-1 increased in MCF-7-LR-2 cells compared with those in MCF-7-Aro cells without E2 treatment. Furthermore, the mRNA expressions of CA2 and GJA1 increased, whereas the mRNA expressions of PGR and CTSD decreased in MCF-7-LR-2 cells versus MCF-7-T cells. Conclusion　The level of ER phosphorylation at Ser-118 increased in a ligand-independent manner, thereby promoting the transcription of target genes in letrozole resistant breast cancer cells, and it plays an important role in letrozole resistance.