Abstarct: Brachytherapy (internal radiation therapy) is a method of treating cancer by means of ionizing radiations in which radioactive sources are placed in or near the tumor to distribute dose effectively into the whole tumor by needles. This important treating method indicates one of the nuclear physics conciliatory applications. 125I is an important radioisotope in nulear medicine that cause to characteristic X and some times an auger effect by means of electron capture which produces low energy between 27 and 35 keV. This radioactive source with short half-life (59.4 days) is usable in patients with low growth rate tumors and utilized in stereotactic operations which are kinds of operations to accurate controlling the radioactive sources into the brain tumors. For using these sources they should be placed in capsules with millimeter scales (as seeds). While brain tumors brachytherapy operation is hard, with respect to close distance between source and tumor, little changes in source locations cause to high changes in dose distributions. So treatment planning has a special importance. Since dimensions and size of tumors in patients are different and accessing the final response with some measurement applications may take times, so we can't access the response in advance. For considering the affect of methods before radiating the patient, we need dosimetry calculating by use of simulation with computers. By determining parameters that explain the source dosimetry characters such as radial dose function and anisotropy function, etc, we can present an optimum effective treatment way for brain tumors. In this study we want to consider 125I source dosimetric parameters and determine radial dose function and anisotropy function in the brain tumor by using MCNPX (2.6.0) simulation code to be aware of dose distributions around the source. Finally air kerma strength for 125I source model 6711 has been evaluated 0.5732cGy.cm^2.h^-1 and dose rate constant was estimated 0.8119cGy.cm^2.U^-1 in the water and 0.657cGy.cm^2.U^-1 in the tumor.