PURPOSE: We investigated the effect of location changes in the inferior border of the belly board (BB) aperture by adding a bladder compression device (BCD). MATERIALS AND METHODS: We respectively reviewed data from 10 rectal cancer patients with a median age 64 years (range, 45~75) and who underwent computed tomography (CT) simulation with the use of BB to receive pelvic radiotherapy between May and September 2010. A CT simulation was again performed with the addition of BCD since small bowel (SB) within the irradiated volume limited boost irradiation of 5.4 Gy using the cone down technique after 45 Gy. The addition of BCD made the inferior border of BB move from symphysis pubis to the lumbosacral junction (LSJ). RESULTS: Following the addition of BCD, the irradiated volumes of SB and the abdominopelvic cavity (APC) significantly decreased (174.3+/-89.5 mL vs. 373.3+/-145.0 mL, p=0.001, 1282.6+/-218.7 mL vs. 1571.9+/-158 mL, p<0.001, respectively). Bladder volume within the treated volume increased with BCD (222.9+/-117.9 mL vs. 153.7+/-95.5 mL, p<0.001). The ratio of irradiated bladder volume to APC volume with BCD (33.5+/-14.7%) increased considerably compared to patients without a BCD (27.5+/-13.1%) (p<0.001), and the ratio of irradiated SB to APC volume decreased significantly with BCD (13.9+/-7.6% vs. 24.2+/-10.2%, p<0.001). The ratios of the irradiated SB volumeand irradiated bladder volume to APC volume negatively correlated (p=0.001). CONCLUSION: This study demonstrated that the addition of BCD, which made the inferior border of BB move up to the LSJ, increased the ratio of the bladder to APC volume and as a result, decreased the irradiated volume of SB.