Boron Neutron Capture Therapy (BNCT) is used as one of treatment methods for deep brain tumors. The nuclear reaction used in this method is 10 B(n,?) 7 Li which has maximum cross section with thermal neutrons. Boron is localized in tumor cells by special drug, then thermal neutrons are absorbed by the boron in the tumor cells and the energy of the reaction products, damage the DNA of the tumor cells. A high intensity neutron source is necessary for BNCT. One of the high intensity neutron sources is nuclear reactor. In order to have a suitable neutron beam for boron neutron capture therapy, we need to design a beam shaping assembly (BSA) for the high intensity neutron source. The BSA should decrease the energy of the neutron to epithermal region, focus the epithermal neutrons on the small place, and suppress the radiation pollution like ?-ray. When the epithermal neutrons beam pass through various tissues of head, it will be converted to the thermal energy range. For evaluating the BSA performance two sets of parameters have been proposed by the International Atomic Energy Agency which are called in-air parameters and in-phantom parameters. This thesis focuses on calculation of in-phantom parameters and dose curves. A designed BSA for the Isfahan MNSR reactor was considered for calculating the parameters in a head phantom. Snyder head phantom in front of the BSA at 0.5 cm was used. We used MCNPX code for simulating the interactions of the neutrons with the phantom and calculating the neutron energy spectrum in different depths. Our calculations showed that the maximum therapeutic gain was 4.7, advantage depth was 10.3 cm, and treatment time was 29 minutes, if boron concentration in the tumor and normal tissue were assumed 65 and 18 ppm, respectively and the maximum absorbed dose rate for normal tissue was 43.4 RBE.cGy/min. We also showed that the most incident epithermal neutrons become thermal after passing through various tissues of the head phantom. Comparison of the calculated in-phantom parameters for Isfahan MNSR reactor with other reactors, shows that the parameters for the Isfahan MNSR reactor have the acceptable values for BNCT.