A new bismuth and polymer-based composite was fabricated to investigate its shielding properties against X-ray in medical applications. Bismuth oxide (Bi₂O₃) were used as the filler (99.999% trace metals basis). The unsaturated polyester (UP) resin and methyl ethyl ketone peroxide (MEKPO) were used as the composite matrix (density was 1.215 g/cm³). The open mold cast technique was used to fabricate Bi₂O₃-polyester resin composite. Silicon rubber was made as a mold formed into open cubes. The dimension of this mold was 4 cm x 4 cm with the thickness of 0.5 cm. The samples were made with different bismuth oxide concentration variations: C1 (0%), C2 (1%), C3 (2%), C4 (3%), C5 (4%), C6 (5%), and C7 (6%). The optical transmission spectra of the bismuth oxide-polyester resin composite of equal thickness x (5 mm) were recorded at room temperature using optical emission spectroscopy (OES, Aurora 4000) GE-UV-NIR in the visible light region. The optical transmission spectra of the composite have been observed in the visible region (400 to 700 nm). According to the results, the samples have different attenuation coefficients. The values are varied related to the sample concentrations. The highest sample concentrations (6%) have 2.680, 2.474, and 2.197 cm^-1, respectively for 40, 50, and 60 kV. Besides, the lowest concentrations show 0.884, 0.827, and 0.811 cm^-1 for 40, 50, and 60 kV, respectively. These results show that the HVL values are varied from 0.26 to 0.85 depending on the X-ray energy and the sample concentrations. The best performance is obtained from C7, indicated by the highest linear attenuation and lowest half-value layer.