Objective To investigate the immunomodulatory effects and the mechanism of interferon (IFN)-γ-pretreated adult autologous adipose tissue-derived mesenchymal stem cells (ADSCs) on peripheral blood lymphocytes. Methods ADSCs were obtained from adult subcutaneous adipose tissues. IFN-γ with and without pretreated ADSCs were used as IFN-γ-pretreated group and IFN-γ-unpretreated group, which were cultured with autologous peripheral blood mononuclear cells (PBMCs) at different concentrations of ADSCs-to-PBMCs ratios in presence of concomitant phytohemagglutinin (PHA)/interleukin (IL)-2 stimulation. After 5 days of culture, the proliferatory inhibitory rate of activated T cells, the percentage of CD4+CD25+ regulatory T cells (Treg), and the expression of indoleamine 2,3-dioxygenase (IDO) mRNA were assessed. Results ADSCs were isolated from autologous adipose tissue, which strongly expressed CD73, CD90, and CD105, as well as displayed adipogenic and osteogenic differentiation. The percentage of CD4+CD25+Treg was significantly higher in IFN-γ-pretreated group than that of IFN-γ-unpretreated group. The expression level of IDO mRNA in ADSCs was significantly increased in IFN-γ-pretreated group than that of IFN-γ-unpretreated group. The proliferation inhibition of activated T cells was significantly decreased in IDO-blocker group than that of IFN-γ-pretreated group (P < 0.01). Conclusion These results suggest that IFN-γ can promote immunosuppressive effects of ADSCs on activated T cells through increased expression of IDO.