Boron Neutron Capture Therapy (BNCT) is an effective and acceptable therapeutic method forthe treatment of brain tumors. This method based on ?? B ( n, ? )? Li reaction. In this method,stable isotope ?? B will be localized into the tumor after infusion into the patient. Then thepatient is irradiated by a beam of epithermal neutrons. The neutrons are changed into thermalneutrons through passing different tissues. Absorption of thermal neutrons in the nuclei ?? B will lead to the production of ? particles and ? Li . The deposited energy from this reactiondistroies boron-labeled tumor cells. Two important factors are boron carrier drug and neutronsource. Nuclear reactors are the best neutron source for BNCT. In order to use these neutronsources for BNCT it is necessary to optimize energy, intensity and contaminations by designinga Beam Shaping Assembly (BSA). The BSA consists of moderator, reflector, collimator andfilters . In this research a BSA has been designed based on a nuclear reactor as neutron sourceusing MCNPX code . Many parameters involve in designing a BSA, including shape, materialand thickness of the material, their order and etc. All these parameters can not be optimizedsimultaneously. Therefore we have to optimize each arameter in a separate stage. Each parameterwill be optimized according to result of its previous stage. The output neutron beamfrom the designed BSA have properties that determined by international atomic energy agency(IAEA). Final design of BSA can be used as a set for boron neutron capture therapy. In orderto have a better analysis from this spectrum,the defined criteria for phantom parameters includingADDR, AD, TG and TT were evaluated. All these parameters have a good condition fortreatment. Dosimetry considerations show that fission neutron spectrum can be used in BNCT.