OBJECTIVE：To study the effects of Lactobacillus fermentum（Lb-f）on inhibition of cytoxan（CTX） on HCT116 cells of human colorectal cancer in vitro and in vivo. METHODS：HCT116 cells were cultured in vitro. MTT assay was used to investigate the effects of 4，2，1 μg/ml CTX and combined with 10 μg/ml Lb-f on the survival rate of HCT116 cells. HCT116 cell xenograft tumor nude mice model was induced to investigate the effects of 100，50，25 mg/kg CTX and combined with 180 mg/kg Lb-f on tumor volume，tumor weight，relative tumor inhibition rate，the sensitization effect of chemotherapy（by q value），the number of peripheral blood leucocyte and platelet. RESULTS：Compared with CTX alone，CTX combined with Lb-f had no significant effect on survival rate of HCT116，relative inhibition rate of tumor volume in nude mice， relative inhibition rate of tumor weight and q value（P＞0.05），but increased the number of peripheral blood leucocyte and platelet（P＜0.05）. CONCLUSIONS： No synergistic effects of Lb-f is found on the inhibition of CTX on the growth of HCT116 in vitro and in vivo；Lb-f can inhibit the decrease of peripheral blood leucocyte and platelet of HCT116 cell xenograft tumor nude mice induced by CTX.