Objective： To investigate how urothelial carcinoma-associated 1 (UCA1) and miR-18a modulates acquired tamoxifen resistance and the relevant mechanisms in estrogen receptor (ER) positive cancer cells. Methods: qRT-PCR was performed to detect UCA1 and miR-18a expression in breast cancer cells. Dual luciferase assay was performed to detect the binding between miR-18a and UCA1 3′UTR. Tamoxifen sensitive MCF-7 cells were transfected with UCA1 expression vector or miR-18a inhi-bitors. Tamoxifen resistant LCC9 and BT474 cells were transfected with UCA1 siRNA or miR-18a mi-mics. CCK-8 assay was performed to detect cell viability. Soft agar assay was performed to assess cell colony formation. Flow cytometric analysis was performed to check cell cycle distribution. Results: UCA1 was significantly upregulated in tamoxifen resistant LCC2, LCC9, and BT474 cells than in tamoxifen sensitive MCF-7 cells. UCA1 expression was significantly upregulated in MCF-7 cells after treatment with 0.1 μmol/L tamoxifen. UCA1 overexpression enhanced cell viability of MCF-7 cells after tamoxifen treatment, while UCA1 siRNA significantly suppressed viability of LCC9 and BT474 cells after tamoxifen treatment. In MCF-7 cells, compared with vector control+tamoxifen group, the average cell colony number and colony size of the UCA1+tamoxifen group was 19.0% more and 29.0% larger respectively, while the proportions of the cells in G1 phase and in S phase were 7.3% lower and 6.7% higher respectively. In BT474 cells, compared with siRNA control+tamoxifen group, the average cell colony number and colony size of the si-UCA1+tamoxifen group were 54.0% less and 42.0% smaller respectively, while the proportions of the cells in G1 phase and in S phase were 9.0% higher and 6.2% lower respectively. UCA1 directly interacted with miR18a and reduced its expression in ER positive breast cancer cells. Knockdown of miR-18a increased viability of MCF-7 cells after tamoxifen treatment, while miR-18a overexpression significantly reduced viability of BT474 cells after tamoxifen treatment. In MCF-7 cells, compared with miRNA inhibitor control+tamoxifen group, the average cell colony number and colony size of the miR-18a inhibitor+tamoxifen group were 15.0% more and 33.0% larger respectively, while the proportions of the cells in G1 phase and in S phase were 8.8% lower and 5.3% higher respectively. In BT474 cells, compared with miRNA control+tamoxifen group, the average cell colony number and colony size of the miR-18a mimics+tamoxifen group were 47.0% less and 25.0% smaller respectively, while the proportions of the cells in G1 phase and in S phase were 13.3% higher and 7.9% lower respectively. Conclusion: UCA1 can increase tamoxifen resistance of ER positive breast cancer cells via competitively inhibiting of miR18a.