To compare the dosimetric difference between three dimensional conformal radiotherapy using the field-in-field (FIF-CRT) and volumetric modulated arc therapy (VMAT) for whole breast irradiation with tumor bed boost after breast-conserving surgery, fifteen female patients with early left-sided breast cancer after breast-conserving surgery were selected. Treatment planning was performed by volumetric modulated arc therapy and three-dimensional conformal radiotherapy using the field-in-field on the same CT, respectively. The target dose distribution, homogeneity and the irradiation dose and volume for the lungs, heart, and contralateral breast were read in the dose volume histogram (DVH) and compared between FIF-CRT and VMAT. The treatment delivery monitor unit was also compared. In comparison with the FIF-CRT planning, the target coverage of PTV1 and PGTV was significantly higher by 5.62% and 10.64% in VMAT. The conformal index and homogeneity of planning target volume by the VMAT planning were better than those by FIF-CRT planning. There were no significant difference in the maximum dose (Dmax) and V10 for the heart and V20 for the left-lung between the VMAT and FIF-CRT planning, but the values of V10, V5, and Dmean for the left-lung; V5 for the heart; V5, Dmean for the contralateral breast and lung; Dmax for the spine cord were increased compared with those by the FIF-CRT planning. The V30, V40 of the left-lung and V20 of the heart by the VMAT planning were lower than those by FIF-CRT. The mean MU of the VMAT was 745 MU, which is significantly higher than that by FIF-CRT planning(250 MU, p<0.001). The VMAT planning improves the dose distribution and homogeneity of PTV for breast cancer radiotherapy after breast—conserving surgery. However, it significantly enlarges the volume of normal tissues irradiated in low dose areas, and increases the MU value in comparison with FIF-CRT．Cited