Background: The immunologic factors are the chief reason for recurrent pregnancy loss (RPL) and induction ofmaternal-fetal tolerance is the main treatment for this cause of RPL, but the effect of this method is uncertainly andneeds multiple doses and/or interventions. The aim of this study was to investigate whether a single administrationof transforming growth factor-β1 (TGF-β1) can improve the pregnancy outcomes of RPL mice and whether theimprovement is cause by TGF-β1 driving the expression of immune tolerance molecule indoleamine 2,3-dioxygenase(IDO).Materials and Methods: In this experimental study, 40 RPL model mice were equally divided into a control group,that received 0.01 M phosphate-buffered saline (PBS), and a treatment group, that received PBS containing 2, 20, and200 ng/ml TGF-β1 via tail vein injection. The mice were sacrificed at 13.5 days of pregnancy and the embryo resorptionrate was determined. The expression of IDO, TGF-β1, and TGF-β3 were detected in the placenta using westernblotting and immunohistochemistry techniques.Results: The expression of IDO was positively correlated with TGF-β1 in the placental tissue of RPL mice (r=0.591,P