Objective To explore immunosuppressive effect and sappan L （WECSL） in heart transplantation of rats. Methods mechanism of watery extract of caesalppinia Wistar rats （donor） heart allografts were transplanted into the abdomen of SD rats （receptor）. Ninety-six SD rats were divided into four groups （24 rats in each group）. Control group： olive oil（8ml/kg·d） treated; group A： cyclosporine A （CsA,5ml/kg·d） treated; groupB： WECSL（37.5g/kg·d） treated; group C： WECSL（25g/kg·d） plus semidose of CsA（2.5mg/kg·d） treated. Median survival time of heart allografts and the histological changes of allografts were examined. Messenger ribonucleic acid （mRNA） of interleukin-2（IL-2）, interleukinf-10（IL-10） in the myocardium were determined by reverse transcription polymerase chain reaction （RT-PCR）, serum level of IL-2 and IL-10 were determined by enzyme-linked immunosorbent assay （ELISA） at 3, 7 days after surgery. Results Compared with control group, median survival time of heart allografts in group A, group B, group C was prolonged （P〈0. 01）, lymphocyte infiltration and myocyte necrosis were relieved, mRNA expression of IL-2 in allografts was lower, mRNA expression of IL-10 was higher （P〈0.01）. The serum levels of IL-2 in group A 3,7days after surgery and in group B, group C 3 days after surgery was lower than that in control group （P〈0.01）. The serum levels of IL-10 in group A 7days after surgery and in group B 3 days after surgery was higher than that in control group （P〈0. 05）. Conclusion Acute rejection of rat heart transplantation can be effectively suppressed by WECSL, Th1 to Th2 polarization induced by WECSL is observed.