A new type of neutron detector based on monocrystalline Si is developed to measure the fiuence and fiux density of thermal and fast neutrons. The principle of this detector is based on the relationship between changes in electrical conductivity and neutron fiuence during irradiation. Therefore, the absolute values of thermal neutron fiuence and fiux density are measured in a facile manner with high reliability. Compared with activation methods,our method not only possesses a similar accuracy, but also demonstrates superior application potential for the investigation of neutron fields in nuclear reactors owing to its suitable half-life.