Abstract: Objective To analyze the set-up errors in the left-right（LR， X axis），superoinferior （SI，Y axis） and anteroposterior （AP， Z axis） directions and the rotations during radiotherapy of prostate cancer in the supine position. Methods Twenty-five prostate cancer patients who received radiotherapy from October 2011 to June 2013 were analyzed. All the patients were treated with intensity-modulated radiotherapy（IMRT） in the supine position after external immobilization using thermoplastic shell. Measurements for set-up errors and rotations in X， Y， Z directions were collected from daily image-guided radiotherapy（IGRT， bony match） pre-treatment. The set-up errors and rotations were analyzed. Results Each patient received 9 fractions of measurement with a total of 225 fractions. Mean set-up errors were（0.19±0.18）cm， （0.36±0.30）cm， and（0.21±0.16）cm in the X， Y and Z axes， respectively. The set-up errors of≥5 mm was 5.8%， 24.3%， and 8.0% of all measured fractions in the X， Y and Z axes， respectively. Mean rotations were （1.07±1.03）°， （0.82±0.66）°， and （0.79±0.68）°in the axis， vertical， and lateral directions， respectively. After comparing the first 5 and the following 4 set-up and rotation errors，there were no significant differences（P＞0.05）. ConclusionThe set-up errors is larger in the Y than in the X and Z axes during radiotherapy of prostate cancer in the supine position. The rotation errors are much smaller and can be omitted.