: Today, cancer is one of the concerns of people. Recently, BNCT treatment has received a lot of attention. This treatment has many features and benefits. The most important feature of this treatment method compared to other methods is the high LET. In fact, in this method, the tumor is treated by delivering the least amount of unwanted dose to healthy tissue. The aim of this study was to evaluate the treatment of breast cancer by BNCT method. The challenge is the dose reached to surrounding healthy tissues, such as the heart and lungs. In this study, MITR neutron beam therapy and MIRD phantom were used. Simulations in this research with the code MCNPX. 0. 2. 7 were performed in two steps. First, a spherical tumor with a radius of 1 cm was designed in the left phantom breast with a concentration of 18:60 ppm in the tumor and healthy tissue. It was then placed in front of a neutron beam. Due to the high dose reaching the heart and lungs, there was a need to find a solution. In the second step, a Beam Shaping Assembly was designed to reduce the dose of fast neutrons and to increase the thermal neutron flux. For BSA, BeO and D 2 O with thicknesses of 1 to 45 cm were used as moderators. Graphite, lead and beryllium with thicknesses of 1cm to 12cm were used as reflectors. The collimiator was designed with graphite, lead and beryllium materials with lengths of 2 cm to 7 cm and finally, D 2 O with a thickness of 5 cm as a retarder, graphite with a thickness of 10 cm as a moderators and graphite with a length of 7 cm as a collimiator were selected. The new narrow lethargy spectrum was calculated. Then the parameters in air and in the phantom for this beam were investigated. The results of this study show that breast cancer can be treated with BNCT with a neutron beam therapy from the designed spectrum.