Reactor dosimetry by the foil activation method and three dimensional diffusion or two dimensional transport calculation have been performed to characterize the neutron field of JOYO. In order to improve the accuracy of the neutron flux calculation for each irradiation test, the JOYO core management code system will be upgraded. In addition, a fine flux distribution inside the irradiation rig was analyzed by the Monte Carlo code. The helium accumulation method has been developed as a neutron fluence monitoring technique to be utilized for long-term irradiation tests. The system was calibrated in the well characterized fast neutron fields of YAYOI.