Abstract: This study analyzed the set-up error by the difference in Body Mass Index (BMI) and applied the error to the existing treatment plan to compare the BMI determined dose for the small intestines and bladder in colorectal cancer to study the usability of belly board. Twelve patients received radiation therapy before or after surgery after being were diagnosed with colorectal cancer between August, 2014 and July, 2015. They were divided into two groups based on BMI (high BMI = 24 kg/m2; low BMI or < 24 kg/m2) and each group was sub classified on the use/ non-use of belly board (Group 1, belly board and low BMI; Group 2, no belly board and low BMI; Group 3, belly board and high BMI; Group 4, no belly board and high BMI). For Group 1, the volume of bladder V10 did not change with V20 2.13±0.83, V30 3.66±2.99 and V40 4.53±3.45 cc. The volume of the small bowel V10 was 1.11±1.30 with V20 2.19±1.82, V30 2.55±2.25 cc and V40 2.75±2.55 cc (Group 2). In Group 3, V10 did not change with V20 11.73±14.94, V3015.08±19.78 and V40 19.40±23.92 cc while the volume of small bowel V10 was 1.19±1.82, V20 16.22±24.36, V30 17.67±25.30 and V40 18.49±26.37 cc. In Group 4, V10 did not change with V20 8.08±9.12, V30 15.45±7.63 and V40 19.53±9.61 cc while the volume of small bowel V10 was 0.73±0.86, V20 7.13±5.38, V30 15.26±14.97 and V40 10.77±7.92 cc. The volume of bladder V10 did not change with 0.39±0.68, V20 12.33±9.98, V30 18.03±8.79 and V40 21.55±8.12 cc while the volume of small bowel, V10 was 18.38±17.00, V20 11.45±1.69, V30 23.67±20.79 and V40 36.17±31.70 cc.