For roughly 25 years, 125 I sources have been used in the treatment of various malignant diseases such as prostate cancer. Three new brachytherapy sources, IR01- 125 I, IR02- 125 I and IR03- 125 I, have been developed and are designed for permanent implant application. The Monte Carlo radiation transport code version MCNP 5 was used to calculate the dosimetry parameters around the sources in accordance with the updated report of the American Association of Physicists in Medicine (AAPM), Task Group No. 43. For each source, the dose rate constant Λ, the radial dose function g L (r), and the anisotropy function F(r, θ), were obtained. The results indicated a dose rate constant of 0.932 ± 0.01, 0.934 ± 0.01 and 0.939 ± 0.01 Gy h −1 U −1 for the IR01- 125 I, IR02- 125 I and IR03- 125 I sources respectively. With the goal of determining an optimal design for a 125 I source, each seed's parameters were compared with other seeds. In this study, the optimal source IR03- 125 I provides the most isotropic dose distribution in water. Finally, the results for optimal source were compared with published results for those of other commercial sources.