Background: Bismuth composites are new and non-lead shields that are capable of breast dose reduction while preserving image quality. In this study, different percentages of Bismuth were used in designing and constructing polymer composites as breast shields in chest CT.  Materials and Methods: Micro particles of Bismuth with a ratio of 1% to 15% in Silicone matrix with two thicknesses were used as Bismuth Silicone composite (BSC) shield. The female chest phantom and standard chest CT exposure factors were used. Dosimetry was performed by thermo luminescent dosimeters. The image quality was evaluated with two methods. Statistical analyses were performed by Kruskal-Wallis and Mann-Whitney tests using SPSS 16. Results: The application of 1% and 15% BSC shields reduced the breast dose to 14.9% and 62.2%, respectively. Increases in image noise in BSC was observed 2.33% (P=0.439) for breast and from 7.13% (p=0.513) for mediastinum areas, depending on the type of shields. Conclusion: The Application of Bismuth shields with Silicone composite and changing of the Bismuth percentages in composite and thickness are effective factors on the breast dose reduction significantly that can reduce the risk of breast cancer when engaged in Chest CT.