Objective To investigate the effect of curcumin an extract of a Chinese medical herb on the sensitivity of CD133 + rectal cancer cells to radiotherapy.Methods In vitro experiments:CD133 +cells were purified with immunomagnetic beads from HRT-18 cell line and divided into curcumin group,radiotherapy group and curcumin plus radiotherapy group.MTT assay and Annexin V/PI staining were used to measure the proliferation and apoptosis of the cells.In vivo experiments:Transplanted rectal tumor was established in 46 nude mice and randomly divided into curcumin group,radiotherapy group and curcumin plus radiotherapy group.Tumor size and apoptosis were detected by daily observation and TUNEL staining respectively.Results Curcumin inhibited proliferation and apoptosis of CD133 + rectal cancer cells when combined with radiotherapy.It also significantly increased the growth inhibition of rectal tumor and promoted the apoptosis of rectal cancer in vivo.MTT assay showed that after 24 hours,compared with that of radiotherapy group(14.6％ ± 1.0％),curcumin plus radiotherapy group (18.7％ ± 1.7％) inhibited the growth of the tumor(P ＜ 0.01).Annexin V/PI showed that curcumin plus radiotherapy group (28.8％ ±3.7％) was significantly different from the radiotherapy group(13.1％ ± 1.4％) in cell apoptosis (P ＜0.01).In vivo,after 6 days,tumor volume (521 ± 79) mm3 in curcumin plus radiotherapy group was significantly lower than that of radiotherapy group(717 ± 134) mm3 (P ＜ 0.01) ; TUNEL staining results indicated that the RCST in curcumin plus radiotherapy group (26.1％ ± 3.3％) were higher than that in radiotherapy group (12.0％ ± 2.1％) (P ＜ 0.01).Conclusions Curcumin significantly enhances the radiosensitizing effect for CD133 + rectal cancer cells.