In order to investigate the antitumor effects of the in vivo G-CSF gene therapy mediated by liposome and its mechanisms, human G-CSF gene was encapsulated into liposome and was directly injected into tumor mass of C26 colon adenocarcinoma-bearing mice. After direct intratumoral injection of liposome encapsulated G-CSF DNA, the subcutaneous tumor growth was dramatically inhibited and the survival time was prolonged significantly. Tumor regression could be observed in about 30%of C-26-bearing mice. By the analysis of the antitumor mechanisms, we found that anti-G41s (600ug/ml) clone could be selected from the tumor cells freshly separated from the treated C-26 tumor mass, and secretion of GCSF in the supernatant could be detected. Northern-blot also confirmed the expression of hG-CSF by the tumor cells. Higher expressions of MHC class I(H-2kd) molecule and ICAM-1 on the tumor cells could be observed. The results demonstrated that liposome can effectively transfect G-CSF gene into tumor cellsin situ, and then increase the immunogenicity of the tumor cells which may contribute to the activation of the local antitumor immune responses effectively.