Abstract: Objective: To demonstrate the selection of breast implant during an immediate breast reconstruction post-mastectomy and analyze the indication of this technique. Methods: From June 2007 to June 2012, a total of 121 patients with breast cancer received immediate breast reconstruction with breast implants. Among the 121 patients, 89 patients had simple mastectomy, while the rest underwent modified radical mastectomy in the Department of Breast Neoplasm, Nanchang No.3 Hospital. The volumes of the resected breast tissues were measured using Archimedes principle. The diameters of the tissues were also determined. Proper breast implants were selected according to the measured data. Results: Postoperative complications, such as implant exposure, flap necrosis, and infection, were not found. Follow-up period ranged from 6 months to 12 months. Patients answered a questionnaire that displayed their degree of satisfaction for the breast operation outcome. Results show that 89.3% of the patients (108/121) were very satisfied, 9% were (11/121) satisfied, and 1.7% (2/121) were unsatisfied. Conclusion: Immediate breast reconstruction with breast implant post-mastectomy is an ideal method for rebuilding the breast. This technique is advantageous because it prevents damage to the donor site and retains the maximal elasticity of the skin for breast reconstruction. Accurate parameters of breast implants, which are important to achieve good surgical results, could be obtained using Archimedes principle.