Oestrogen receptor a (ERa) is overexpressed in two-thirds of all breast cancers and involves in development and breast cancer progression. Although ERa-positive breast cancer could be effective treated by endocrine therapy, the endocrine resistance is still an urgent clinical problem. Thus, further understanding of the underlying mechanisms ERa signalling is critical in dealing with endocrine resistance in breast cancer patients. MCF-7 and T47D breast cancer cell lines are used to carry out the molecular biological experiments. Western blot is used to assess the relative protein level of ERa, RNF168 and actin. Real-time PCR is used the measure the relative ERa-related gene mRNA level. Luciferase assay is used to measure the relative ERa signalling activity. Chromatin immunoprecipitation is used to measure the RNF168 binding affinity to ERa promoter regions. WST assay and flow cytometry are used to measure the cell proliferation capacity. We use Student's t test and one-way ANOVA test for statistical data analysis. Here, we report an important role in ERa-positive breast cancer cells for RNF168 protein in supporting cell proliferation by driving the transcription of ERa. RNF168 is highly expressed in breast cancer samples, compared with normal breast tissue. In patients with breast cancer, RNF168 expression level is correlated with poor endocrine treatment outcome. Depletion of RNF168 causes decreased cell proliferation in MCF-7 and T47D cells. Besides, depletion RNF168 reduced mRNA level of ERa and its target genes, such as PS2 and GREB1. Chromatin immunoprecipitation revealed that ERa transcription is associated with RNF168 recruitment to ERa promoter region, suggesting that transcriptional regulation is one mechanism by which RNF168 regulates ERa mRNA level and ERa signalling in breast cancer cells. RNF168 is required for ERa-positive breast cancer cell proliferation and facilitate ERa signalling activity possibly through promoting transcription of ERa.