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To investigate the dosimetry advantages of voluntary deep inspiration breath-hold (V-DIBH) technique using optical surface monitoring system(OSMS) for whole breast irradiation (WBI) in left breast cancer after breast conserving surgery(BCS), verify the reproducibility and acceptability of this technique. Materials/Methods: Twenty patients with left breast cancer who underwent WBI after BCS were enrolled in a prospective phase 2 Study. Simulated CT scans were performed in both Free Breathing (FB) and V-DIBH for all patients. WBI plans were designed and the volume and dose of heart, left anterior descending coronary artery (LADCA) and lung were compared between FB and V-DIBH. Cone beam CT (CBCT) was performed for the first 3 treatments, then weekly during OSMS-assisted V-DIBH treatment to evaluate the accuracy of OSMS technique. The acceptance of this technique was evaluated with in-house questionnaires completed by patients and radiotherapists. Results: The median age of the patients was 45ys (27-63ys). All patients received WBI with simplified IMRT technique to a total dose of 43.5Gy/ 2.9Gy/15f. 17 of 20 patients received concomitant tumor bed boost to a total dose of 49.5Gy/3.3Gy/15f. The volume of left lung increased significantly from FB to V-DIBH (1294AE218 ml vs. 2037AE483 ml, P<0.001). Compared with FB, there were significant decrease in mean dose to the heart (262AE163 cGy vs 515AE216 cGy, P<0.001), LADCA (1191AE827 cGy vs 1794AE833 cGy, P<0.001), and V20 of left-lung (15.5AE3.2% vs 20.7AE4.0%, P<0.001) for V-DIBH. The median delivery time of radiotherapy treatment was 4 minutes (1.5-11 minutes). The median deep breathing cycles was 4 times (2-9). After OSMS assisted setup, the median shift based on CBCT in the three translational directions (vertical, longitudinal, and lateral) was 0.16AE0.22cm, 0.05AE0.30cm, and 0.05AE0.20cm. The average score of acceptance of V-DIBH by 20 patients was 8.65AE0.9 (full score 12 points). 10.6AE3.2 (full score 15 points) by 9 radiotherapists which showed good acceptance. Conclusion: V-DIBH technique in left breast cancer patients for WBI after BCS significantly reduces the cardiopulmonary dose compared with FB. OSMS-assisted V-DIBH is accurate and feasible with good acceptance by both patients and radiotherapists.