Objective To explore the effect of cytotoxic T lymphocyte-associated antigen 4 (CTLA4-Ig) fusion protein on the function of orthotopic liver allograft. Methods Orthotopic liver allograft models of rhesus monkeys were established in this study. The survival time, liver function and morphologic changes of graft were observed, respectively. The levels of IL-2 and IL-10 were detected by ELISA. Apoptosis was monitored by TUNEL.Results The average survival of control group was 6.57 d, while the average survival of CTLA4-Ig group was 14.92 d, which was statistically prolonged (Plt;0.05). Serum ALT level was highly increased, and Alb level decreased obviously in control group. While the levels of ALT and Alb kept in normal in CTLA4-Ig group. After day 3-7, the expressions of IL-2 were highly expressed in control group, while the expressions of IL-10 in CTLA4-Ig group were higher than those of control group. The severity of rejection reaction after day 3 was weaker in CTLA4-Ig group than that of control group by histological assessment. The apoptosis index after day 3 in the liver cells was highly increased in control group as compared with the CTLA4-Ig group. Conclusions CTLA4-Ig fusion protein therapy can induce immunotolerance and prolong the survival of recipients. The increasing of cytokines IL-2 or the decreasing of cytokines IL-10 may be one of the laboratory indexes in monitoring immunotolerance of transplantation.