With the fixed jaw (constant field width) mode of helical tomotherapy, the craniocaudal dose penumbra is a concern; this problem can be partly solved by using a narrower jaw width, but treatment time becomes much longer. More suitable compromise between craniocaudal dose spread and treatment time may be the use of the recently developed dynamic jaw technology. The dynamic jaw mode can be used in conjunction with both helical and static tomotherapy delivery modes. The purpose of this study was to investigate the efficacy of the dynamic jaw mode for static tomotherapy of breast cancer. Treatment plans of the whole breast and supraclavicular regional lymph nodes, and plans for the whole breast only were generated with the static tomotherapy delivery mode in 10 left-sided postoperative breast cancer patients. Whole breast and supraclavicular nodal radiation therapy was planned with six gantry angles. Whole breast radiation therapy was planned with two gantry angles. Plans with a field width of 2.5 or 5 cm with the dynamic or fixed jaw modes were made for each patient. The prescribed dose was 50 Gy in 25 fractions. Delineated organs at risk (OARs) were the ipsilateral and contralateral lungs, esophagus, spinal cord, heart, contralateral breast, stomach, skin, larynx, thyroid, trachea, and all of the healthy tissues. Plans were compared in terms of planning target volume (PTV) doses, homogeneity, conformality, doses to OARs, and the treatment time. No significant differences were seen in any of the PTV coverage, overdosage, homogeneity, and conformality. In whole breast and supraclavicular nodal radiation therapy, mean doses of the larynx, skin, and all healthy tissues were lower in the dynamic jaw mode than in the fixed jaw mode with a 5-cm field width (p < 0.01). Mean doses of the larynx and skin were lower in the dynamic jaw mode than in the fixed jaw mode with a 2.5-cm field width (p < 0.05). In whole breast radiation therapy, mean doses of the skin and healthy tissues were lower in the dynamic jaw mode than in the fixed jaw mode with a 5-cm field width (p < 0.05). Mean doses in any of the OARs were not higher in the dynamic jaw mode with a 5-cm field width than in the fixed jaw mode with a 2.5-cm width in both whole breast and supraclavicular nodal radiation therapy and whole breast radiation therapy. The use of a 5-cm width instead of 2.5 cm reduced the treatment time by approximately 40%. The use of the dynamic jaw mode instead of the fixed jaw mode prolonged the treatment time by approximately 7%. The dynamic jaw mode provided better sparing of OARs without affecting PTV homogeneity and conformality for postoperative breast irradiation with static tomotherapy. Considering the treatment time, the dynamic jaw mode with a 5-cm field width is more efficient than the fixed jaw mode with a 2.5-cm width.