Background and purpose: More than 90% of cancer patients are incurable because of drug resistance. Activation of PI3K/Akt/mTOR signaling pathway in breast cancer, as a target for chemotherapy drugs has become a hot topic of breast cancer treatment. This study aimed to investigate the effect and mechanism of XCL1 on the proliferation of drug-resistant breast cancer cell, whether is related with the mTOR signaling pathway. Methods:Established gemcitabine-resistant breast cancer cell lines (MDA-MB-231/Gem). CCK8 to detect the proliferation of MDA-MB-231 and MDA-MB-231/Gem, RT-PCR and ELISA to determine the XCL1 expression level of the two cell lines, Western blot to detect the expression of mTOR. Results:Compared with MDA-MB-231, MDA-MB-231/Gem showed an enhanced proliferative capacity. The expression of XCL1 was increased in the resistant cell lines. Both of protein level and phosphorylation level of mTOR increased in drug-resistant cell lines. The MDA-MB-231 added exogenous XCL1 for 24 h, showed an enhanced cell proliferation. Adding anti-XCL1 antibodies in MDA-MB-231/Gem could reduce cell proliferation and treating MDA-MB-231/Gem with the mTOR inhibitor could also reduce cell proliferation, as well as the XCL1 expression level. Conclusion:XCL1 promotes the proliferation of drug-resistant breast cancer cells mediated by activation of the mTOR pathway.