Objective To evaluate quantitatively the extra absorbed dose generated by image guided system of the Cyberknife.Methods The exposure parameters and the average frequency of images acquisition during the treatments with G3 Cyberknife in 300 cases with various tumor locations and tracking modes were collected for analysis ;The measurements of the absorbed doses in the phantom with 5 points after single exposure generated by image guided system in various exposure parameters were performed with equipment of phantom CIRS Model 002LFC and 002 PRA with ion chamber PTW30010 included.Based on the data we collected andmeasured,the conclusion of how much extra absorbed dose generated by image guided system of Cyberknife the patients received during Cyberknife treatments was drawn.Results With an average fractions of 3.94(the median was 4),the average exposure frequency was 36.5 times for static tracking per patient per fraction,while it was 49.2 times for dynamic tracking.The experimental results with various exposure parameters,positions and tissue densities showed that the minimum absorbed dose after single exposure was 0.5 μGy,while the maximum was 385 μGy.Conclusions The image guided system of the Cyberknife can induce quite low absorbed dose in patient,while providing all three types of image guided function.