Abstract: :Objective 　To evaluate the dosimetry of electron beam with the use of computed tomography (CT) based radiotherapy on chest wall for postmastectomy patients. Methods 　CT simulation was conducted for 20 breast cancer patients having received electron beam chest wall irradiation after postmastectomy, and digitizing reconstitution of the image pictures f rom spiral CT equipment and outlining the clinical target volume (CTV) and organs at risks (OARs ) such as heart and lung, calculating their volume and exposure dose, were done by the 3D treatment planning system ( TPS) . The prescribed dose for chest wall was 5 000 cGy . Results 　The maximum dose (Dmax) 、mean dose (D mean) and dose of 90 % CTV (D90) for the left breast cancer were (5 542 ± 279) cGy、(4 809 ±116) cGy and (4 526 ±239) cGy respectively. The Dmean and percentage of volume receiving more than 20 Gy (V20) and 10 Gy (V10) of left lung were (1 616 ±608) cGy、(33 ±13) % and (44 ±16) % respectively. The mean dose (Dmean) and volume receiving more than 30 Gy (V30) of heart were (1 008 ±457) cGy and (13 ±9) %. The Dmax、Dmean and D90 for the right breast cancer were (5 554 ±253) cGy、(4 783 ±89) cGy and (4 496 ±101) cGy respectively. the Dmean、V20 of right lung were (1 416 ±567) cGy、(30 ±12) % respectively. Conclusion 　By the use of computed tomography (CT)-based to perform the plan of electron-beam radiotherapy on chest wall, we can understand precisely the dosimetry of the target volume and normal structures, and perfect the radiation plan better.