Objective To compare the dosimetry difference between helical tomotherapy and fixed field IMRT for breast cancer radiotherapy after breast-conserving surgery. Methods Ten patients received radiotherapy after breast-conserving surgery were selected. For each patient, two treatment plans (FF-IMRT plan and HT plan) were designed with Eclipse and TomoTherapy. Same institutional dose-volume constraints for breast cancer were used in both techniques. Targets and organs at risk were compared with paired t-test for two planning. Results In comparison with the FF-IMRT planning, PTV coverage of HT plan group increased, D_mean became lower (P=0.000,0.002). the HI and CI were significantly higher for HT (P=0.001,0.023).Compared with the FF-IMRT plans, V₅,V₁₀,V₂₀,V₃₀,V₄₀ and D_mean of the ipsilateral lung were reduced by about 28%,30%,35%,46%,61% and 32%(P=0.000),those of full lung were reduced by about 30%,28%,34%,46%,62% and 26%(P=0.000), those and D_max of heart were reduced by about 57%,59%,71%,82%,91%,45% and 12%(P=0.000—0.002).The D_mean and D_max of contralateral breast were higher by 0.9 Gy and 3.0 Gy (P=0.000,0.000) respectively, those of contralateral lung were higher by 0.42 Gy and 2.70 Gy (P=0.000,0.000) respectively. It takes significantly long time for the HT plans designed than FF-IMRT plan (11.98 min:5.96 min,P=0.000). Conclusions The HT planning improve the HI and CI of PTV, organs at risk can get equivalent or superior dose distribution compared with the FF-IMRT technology, but it is feasible for clinical applications in breast-conserving surgery as a new method.