Background: Neoadjuvant chemotherapy has gradually become an important means of breast cancer treatment; however, tumor regression following chemotherapy remains a concern. This study was conducted to investigate the effect of ultrasound-assisted carbon nanoparticle labeling in neoadjuvant chemotherapy for breast-conserving surgery in breast cancer.Methods: This was a prospective clinical trial study (clinical registration number: ChiCTR-OOC-15006844). Sixty-eight breast cancer patients confirmed by biopsy between July 2015 and January 2017 were randomly selected from the clinical data. Of these, 32 patients were screened for neoadjuvant chemotherapy, forming a consecutive, random series. An ultrasound-guided carbon nanotube was used to mark the original tumor, and sentinel lymph node biopsies were performed. After 4-6 cycles of standard neoadjuvant chemotherapy, 26 patients were selected for breast-conserving surgery. The feasibility and validity of carbon nanoparticle labeling were analyzed through the negative rate of incision margin, the volume of resected tumors, the detection rate of black-stained sentinel lymph nodes, the recurrence rate of ipsilateral breast, and postoperative survival.Results: In all, 32 patients underwent sentinel lymph node biopsy, 29 cases were detected (90.6%), the false-negative rate was 3.8% (1/26), and 0-4 sentinel lymph nodes (mean 1.8±1.1) were detected. A total of 26 patients underwent breast-conserving surgery, 5 underwent secondary excision, and 1 underwent subcutaneous adenectomy due to a positive margin. The minimum margin between the resected site and the infiltrated part was 1.0-2.1 cm (1.3±0.3 cm). The diameter of resected tumors ranged from 2.2 to 4.5 cm (3.1±0.6 cm). No recurrence or distant metastasis of ipsilateral breast tumors was observed during follow-up (the median follow-up time was 9 months).Conclusions: Ultrasound-assisted carbon nanoparticle labeling is effective for sentinel lymph node tracing before neoadjuvant chemotherapy and has a high detection rate for metastatic lymph nodes. During breastconserving surgery, it can determine the extent of tumor resection to achieve precision surgical treatment.