Background: We aimed to assess the effect of the DIBH plan on cardiac and other organs at risk received dose during radiotherapy in left breast cancer patients. Materials and Methods: The study was carried out on 30 patients with left breast cancer with a history of mastectomy/lumpectomy surgery who were referred to the radiotherapy department of the Cancer Institute of Iran. Each patient underwent computed tomography (CT) simulations in two respiratory phases, including deep inspiration breath-hold (DIBH) and free-breathing (FB). In addition, the dose-volume histograms (DVHs) of the heart, lung, spinal cord, and breast of each respiratory phase were compared. Results: We observed a significantly higher mean of heart dose in FB in both lumpectomy and mastectomy groups (P value<0.05). We also compared the means of V25 and V30 heart between FB and DIBH—for both, the received dose was statistically higher in FB than DIBH. The mean dose received by the lung and spinal cord was higher in FB than DIHB. However, the observed difference was only significant in the lumpectomy group (P value<0.05). Conclusion: The DIBH is a viable method that could be suggested to reduce the mean dose of the heart during left breast cancer radiotherapy.