Intraoperative radiotherapy or brachytherapy is a kind of treatment in which the radiation source (a capsule containing certain gamma-emitting element such as palladium-103, cesium-137, and iodine-125) is implanted inside the body close to the diseased (cancerous) tissue. The source proximity to the tumor increases the dose absorbed in cancerous cells and kills the cells. Brachytherapy is a therapeutic approach for prostate cancer using a maximum amount of 1 mCi as the source activity. The activity of iodine-135 radioisotope source implanted in the prostate gland is 0.3-0.8 mCi with a half-life of 59.4 days. The purpose of this study was to calculate the uncertainty of the dose received by healthy and cancerous prostate tissue. In this study, the MCNPX 2.6 code and the TG-43U1 protocol were used to simulate the source. The ORNL phantom was applied for prostate dosimetry. The brachytherapy source I-125 (Model Amersham Health 6711) with an activity of 0.5 mCi was simulated in this study. This source consists of a capsule (4.5 mm) containing a cylindrical silver indicator with a mass density of 10.5 g/m 3 . The indicator was coated with a complex of radioactive iodine source (Br 5 I 2 ) with a thickness of 2.0 µm. The dosimetric parameters of the source simulation were calculated to determine its accuracy. Dosimetric parameters included air kerma strength, dose rate constant, geometric function, beam dose function, and anisotropy function. The air kerma quantity was 0.663 cGy.cm 2 /h.mCi and dose rate constants in two phantoms of liquid and solid water were 0.962 cGy/h.U and 1.02 cGy/h.U, respectively. The ORNL phantom was used to simulate the body and prostate gland. In order to calculate the equivalent doses and uncertainties of healthy and cancerous prostate, two different treatments were used for the source implantation in the prostate. In the first therapeutic plan, 84 sources were placed at three different volumes of the prostate. Dose value ??in the prostatic tissue with the volume of 52.01 cm 3 and a point (granular) source with the activity of 0.5 mCi was calculated as 95.555 (95.583 Gy). 9.63% inflation in the prostate volume caused by the source implantation, the tissue cumulative dose showed the amount of 91.556 (91.586 Gy) in the point (granular) source. Maximum uncertainty of the tissue cumulative dose was 25.13% in this plan. In the next stage, 76 sources were implanted at different volumes of the prostate. Healthy and cancerous doses were obtained as 96.310 Gy and 128.425 Gy, respectively, with an output uncertainty code of 0.01% at a volume of 30.02 cm 3 . The prostate gland swells as a result of implanting the sources based on the treatment plan, leading to displacement of the inserted sources. In this case, the cumulative equivalent doses in healthy and cancerous tissues as well as the uncertainty in the source displacement with the granular sources were 39.180 Gy (uncertainty of 2.75%) and 109.438 Gy (uncertainty of 13.84%), respectively. A maximum uncertainty of ± 20% was obtained at this stage. Considering the simultaneous effects of granule displacement and a 12% prostate inflation following 30 days of implantation, the cumulative dose of healthy and cancerous tissues were achieved as 29.454 Gy (uncertainty of 4.60%) and 101.747 Gy (uncertainty of -2.90%) for granular sources with a maximum uncertainty of 0.01% in Monte Carlo calculations.