Abstract: ObjectiveTo investigate the optimal alignment methods for the treatment of abdominal tumor in IGRT. Methods 20 abdominal tumor patients were treated with the Elekta Synergy IGRT system.KV-CBCT images received before every treatment fraction.The difference in bone alignment，grey value alignment and manual alignment was compared. Results Total of 282 sets of CBCT images were analyzed for 20 abdominal tumor.The mean±standard deviation in the X，Y and Z axis directions were (-0.02±0.27)cm,(-0.04±0.28)cm,(-0.03±0.27)cm; (0.08±0.46)cm,(0.11±0.48)cm,(0.09±0.48)cm;(-0.01±0.28)cm,(-0.02±0.28)cm,(-0.02±0.28)cm,respectively,in abdominal tumor with bone alignment，grey value alignment and manual alignment．The mean±standard deviation in the X，Y and Z rotation directions were (0.57±1.78)°,(0.73±2.17)°,(0.67±1.98)°; (0.15±1.24)°,(0.12±1.51)°,(0.23±1.47)°; (0.05±0.92)°,(0.02±1.02)° and (0.18±1.04)°, respectively,in abdominal tumor with bone alignment，grey value alignment and manual alignment．There was no significant difference between bone alignment and grey value alignment，but manual alignment was different from them. Conclusion There exists some extent of setup error in 3DCRT or IMRT of abdominal tumor patients.Which alignment can be chose based on the diseased region .It is suggested that manual adjustment after bone alignment is necessary.