A series of spectral tailoring irradiation tests for type 304 stainless steel (304SS) has been completed in the Japan Materials Testing Reactor (JMTR). The purpose of the experiments is to study the material property changes by irradiation with different neutron energy spectrum. A new procedure of neutron dosimetry and the verification results of neutronics calculation are described here. According to the procedure, the detailed estimation with neutron spectrum uncertainties at an actual neutron field is made possible to determine accurate dpa rate and helium production rate.