Objective To study the difference in dosimetric characteristics and protection of organs at risk（OAR）between helical tomotherapy（HT） and routine intensity modulated radiation therapy（IMRT） for breast cancer patients after breast-conserving therapy. Methods CT images of 10 breast cancer patients after breast-conserving therapy were delineated and transmitted to HT planning and Varian Eclipse systems. Planning target volume（PTV） and primary lesion（PGTV） were contoured on CT slices of each patient.Prescribed dose for PTV was 50Gy／25f，while dose for PGTV was boosted to 60Gy／25f. Homogeneity index（HI），conformity number（CN）， isodose line and dose volume histogram（DVH） were used to evaluate the dose distribution in tumor and OARs. Results The dose distribution HI and CN of HT plan were better than those of IMRT（P＜0.05）. Compared with IMRT，HT notably reduced the volume of ipsilateral lung at 5，10，20 and 30Gy（P＜0.05）. The mean dose of whole lung also was reduced（P＜0.05）. But the volume of contralateral lung at 5Gy was increased in HT. The HT plan was better for the protection of heart in patients with over than the IMRT（P＜0.05）. Conclusion The two treatment modalities can satisfy clinical requirements. HT has a better dose distribution and protective effect for OARs than IMRT.