Objective To explore the regulation mechanism of over-expression of circular RNA-103809(circRNA-103809) on oxidative stress and immunity of breast cancer cells. Methods The pcDNA empty plasmid and circRNA-103809 over-expression plasmid were transfected into breast cancer cells MCF-7, which were included into negative control group and circ103809 group, respectively, and the blank control group was set up. The expression of circ103809 in each group was detected by real-time fluorescence quantitative polymerase chain reaction(RT-PCR). The cell growth was detected by clone formation assay. The oxidative stress markers [superoxide dismutase(SOD), malondialdehyde(MDA), glutathione(GSH)] were detected by biochemical kits. The changes of mitochondrial membrane potential(MMP) were detected by flow cytometry. The expressions of proliferation-related proteins(Ki67, p21), proliferating cell nuclear antigen (PCNA), B lymphoma-2 gene(Bcl-2), Bcl-2 associated X protein(Bax) and cell proliferation gene c-Myc were detected by Western blot. The levels of inflammatory factors (IL-6, IL-10) and inducible nitric oxide synthase(iNOS) were detected by enzyme-linked immunosorbent assay(ELISA) and qRT-PCR, and p65 content was detected by immunofluorescence. Results Compared with blank control group and negative control group, green fluorescence signals were enhanced in circ103809 group, relative expression levels of circ103809 and p21 protein, levels of GSH and MDA, Bax/Bcl-2 and IL-10 were significantly increased. Formation rate of cell colony, relative expression levels of Ki67, PCNA and c-Myc proteins, SOD, IL-6, iNOS and positive rate of nuclear p65 were significantly deceased(P<0.05). Conclusion Over-expression of circRNA-103809 can inhibit the proliferation of breast cancer cells MCF-7, which may be related to promoting oxidative stress level and alleviating inflammation response.