Neutrons are produced in accelerators by irradiating heavy targets with an electron or proton beam. Produced neutrons are of high energy. The purpose of our work is optimization the neutron flux by MCNP-6 code for production the thermal neutron flux using a moderator and for convertation the fast neutron flux into a thermal neutron flux. In this article, we are interested in a thermal neutron flux due it is useful for method of neutron activation analysis. In conventional sources, the moderator is usually a large volume of water or paraffin around the source. Initially, fast neutrons have energy above 1 MeV, and then slow down to energies below 1 eV.