Uranium-235 ( 235 U) is an isotope of Uranium making up about 0.715% of natural Uranium. Unlike the predominant isotope Uranium-238, it is fissile, i.e., it can sustain a fission chain reaction. In this isotope, fission occurs by thermal neutrons. The usage of fission is extracting extremely high energy. In any fission of 235 U by thermal neutron, in addition to fission fragments, several fast neutrons are released, which can result new fission events. The fission fragments have excess neutrons that remove these excess neutrons through the emission of one or more neutrons at the instant of fission (within 10 -l7 s). These neutrons are known as prompt neutrons, the major of neutrons from fission are prompt neutrons (excess of 99%). In addition to the prompt neutrons, other neutrons called delayed neutrons are emitted in the fission process being released in about one minute after fission. These neutrons are emitted following the beta decay of the fission fragments. The total intensity of delayed neutrons is about one neutron per any one hundred fissions, but these delayed neutrons are very important in the control of nuclear reactors. In this research, the grade or concentration of unknown Uranium ore, was measured by irradiation of unknown Uranium ore sample in Isfahan Miniature Neutron Source Reactor (MNSR) by counting its delayed neutrons and comparison of the result with delayed neutrons count of standard sample. The BF 3 detectors were used for delayed neutrons count. These detectors are able to count thermal neutrons. Delayed neutrons have mean energy about 0.4 MeV. These neutrons must be moderated for counting. Designing and optimization of a moderator for counting system were done using MCNPX Code. The unknown and standard samples were irradiated in different times and their delayed neutron counts were recorded in two minutes. Since the number of delayed neutrons, counted by two separated BF 3 detectors, in the standard sample was twice as many as it counted in the unknown sample during the first two minutes, it was concluded that the grade of Uranium in the standard sample was two times as mush as in the unknown sample. The grade of Uranium in the Uranophane ore sample, which was considered as the standard sample, was calculated through atomic spectroscopy and Gamma spectroscopy (specific activity) methods.