Abstract: Three scatter calibration factors of ⁶⁰Co therapy unit were calculated by using Monte-Carlo method. The effects of radiation field and depth to total scatter calibration factor S_c,p and the phantom scatter calibration factor S_p of ⁶⁰Co therapy unit were also calculated. The calculating results agree well with the measuring results. S_c,p and S_p increase with the increasing of the fields. S_c,p and S_p have increasing trend with depth increasing while beam field larger than 10 cm×10 cm. S_c,p and S_p have decreasing trend with depth increasing while beam field smaller than 10 cm×10 cm. Scatter calibration factor is not be overlooked in calculating the prescription dose. Monte-Carlo method is able to establish a set of accurate and comprehensive scatter calibration factor, and provides the basis of clinical use, quality assurance and quality control for radiotherapy.