The purpose of this study was to clarify the relationship between ovarian cancer peritoneal dissemination and indoleamine 2,3-dioxygenase (IDO) expression, and to explore the possibility of IDO-targeting molecular therapy for ovarian cancer. We transfected an IDO expression vector into the IDO-non-expressing human ovarian cancer cell line OMC-1, and established an IDO-expressing cell line (OMC-1/IDO) to examine the relationship between IDO expression and cancer cell growth in vitro and in vivo. IDO expression did not influence cancer cell growth and invasion in vitro, but promoted tumor growth and peritoneal dissemination in vivo. Immunostaining showed that IDO expression inhibited natural killer (NK) cell accumulation in tumors and promoted tumor angiogenesis. In addition, the oral administration of the IDO inhibitor 1-methly-tryptophan inhibited the growth of OMC-1/IDO-derived subcutaneous tumors in mice. These findings indicate that IDO promotes the peritoneal dissemination of ovarian cancer by inhibiting NK cell accumulation in tumors and promoting angiogenesis, supporting the applicability of IDO-targeting molecular therapy in ovarian cancer.