Abstarct: Brachytherapy is radiotherapy method in which special radioactive sources are used for therapy by using them in close distance with, in contact with tumor by implanting them in tumor. In this method, a high dosage is applied to tumor area, in such a manner that healthy tissues around tumor receive the lowest dosage. 103pd and 125I sources are widely used in treatment of malignant tumors. In 1986, 103pd source was introduced as a good alternative 125I. This source was first used in brachytherapy of breast cancer in Toronto's Sunnybrook regional cancer center, Canada, and is currently used in treatment of prostate cancer, breast cancer, and aggressive malignant tumors. Since dosage declines in a short distance from 103pd, therefore, by proper regulation of dosage rate, one can reach a high efficiency in killing cancer cells in rapidly growing tumors. Therefore, it is important to determine dosimetry parameters of these sources before technical application of them. This is only possible only by experimental measurement baxsed on dosimetry of brachytherapy. Due to gradient high dose near source and low dose in high distances from source, experimental dosimetry of brachytherapy is complicated, and it can even be said that it is impossible in short distances. One of the techniques used to solve this problem is use of Monte Carlo simulation. Calculation of dose distributions in short distances as well as validation of experimental results is conducted by powerful codes such as MCNP, BEAM EGSNRC, PENELOPE, GENT 4 and ENTRANT/TS. In 1995, The American Association of Physicists in Medicine (AAPM) poplished a protocol TG-43 which included a new formulation for calculation of dose of brachytherapy sources. This protocol included application of Monte Carlo in simulation, instruction for Monte Carlo dosimetry, recommendations for simulator and validation of calculations. According to this protocol, dosimetry parameters including Air Kerma strength, Dose Rate Constant, Geometry Function, Radial Dose Function, Anisotropy Function, with each of these quantities contributing to determining absorption dose reached tumor. Because it is impossible in practice to conduct directly in human body the test of accuracy of dose reached to region under treatment, simulation for determining dosimetry parameters in conducted in water phantom using close match of density and compounds existing in water with body tissue. In this research, in addition to determining dosimetry parameters according to Tg-43 instruction using water phantom, these parameters were also calculated in phantom of body tissue, and by comparing the results from these two states, the extent to which values from simulation by water phantom may be used instead of body tissue to treat malignant tissue was evaluated