At the Kyoto University Critical Assembly (KUCA), critical irradiation experiments on 237Np and 241Am foils are carried out in the neutron hard spectrum core. For nuclear transmutation of minor actinides, special attention is paid to determining 237Np and 241Am fission reaction rates and to 237Np capture reaction rates in the KUCA hard spectrum core. In the back-to-back (BTB) fission chamber, two nuclide foils (test: 237Np or 241Am; reference: 235U) are set closely to each other to measure the aforementioned fission reaction rates. Interestingly, the experimental 237Np and 241Am fission and capture reaction rates are attained by critical irradiation at low W reactor power for 1 h in the core and are successfully deduced through signals from the BTB fission chamber (fission) and the gamma-ray detection (capture) after the irradiation, respectively, together with MCNP calculations.