Abstract: Objective To assess the effect of bellyboard on volume and dose of small bowel in post-operative rectal cancer patients treated with three-dimensional conformal radiotherapy. Methods Eight patients with rectal cancer after radical operation were examined by CT scan.. Two sets of images were taken for the whole pelvis with or without bellyboard. By a 3D planning system a three-field technique was planned. The max dose and dose-volume of small bowel at the 20Gy,30Gy,40Gy,45Gy dose level(V20,V30,V40,V45) wereanalysed in the plan of two positions. Results 　Of the two set s of plans, the volume of the plan with bellyboard was smaller than without bellyboard at the level V20, V30, V40 for small bowel. However, the max dose and V45 of the plan with bellyboard was not significantly lower than without bellyboard. Conclusion 　Using bellyboard could signicantly reduce the irradiated volume of small bowel for post2operative rectal cancer patient when t reated with three2dimensional conformal radiotherapy.