Cytotoxic T lymphocyte (CTL) response is a critical component of the immune response to tumors, therefore optimal induction of CTL responses to tumor antigens is highly desired for developing efficient cancer immunotherapy. IL-27 is a member of the IL-12 family of cytokines that is comprised of an IL-12 p40-related protein subunit, EBV-induced gene 3 (EBI3), and a p35-related subunit, p28. IL-27 functions through IL-27R and has been shown to have potent anti-tumor activity via activation of a variety of immune components, including anti-tumor CD8+ T cell responses. However, the exact mechanisms of how IL-27 enhances anti-tumor CD8+ T cell responses are not fully understood. In this paper we mainly discuss the evidences that suggest novel mechanisms by which IL-27 enhances anti-tumor CTL responses, including IL-27 inhibition of activation-induced cell death; the phenotypes of IL-27-stimulated CTLs; IL-27-induced CTL IL-10/IL-21 production and IL-27-mediated suppression of regulatory T cell responses. These evidences suggest that IL-27 may have a great potential to be utilized in boosting anti-tumor CTL responses in human cancer patients.