MuS110 and MT110 are BiTE antibodies bispecific for CD3 and EpCAM, which is the most frequently and highly expressed tumor-associated antigen on breast cancer. And pronounced expression of IDO1 has also been reported in breast cancer. Our study aimed to investigate whether IDO1 inhibitor D-1MT combing with MuS110/MT110 had synergistic antitumor effects on IDO expressing EpCAM-positive breast cancer cells in vitro and in vivo. Data suggested that the expression of IDO1 on Epcam-positive breast cancer 4T1 and MCF-7 decreased MuS110/MT110 antitumor efficacy by the suppression of T cells activation in vitro. Combining D-1MT with MT110 in IDO+MCF-7 cells, or with MuS110 in IDO+4T1 cells, significantly improved the antitumor efficacy of BiTE antibodies via increasing T cell cytotoxicity and contributing to cytokines releasing. In vivo assay, combination of D-1MT with MT110 in NOD/SCID mice bearing IDOhi MCF-7 xenografts or MuS110 in immune competent BALB/c mice bearing IDOhi 4T1 xenografts suggested the similar synergistic effect. Together, IDO inhibition could reverse the suppression of T cells due to IDO expressing on breast cancer, and improve the antitumor efficacy of EpCAM/CD3-bispecific BiTE antibody.