AbstractNeutron spectrum measurements in a cylindrical assembly of 316L stainless steel were carried out for DT neutrons to assess the multigroup neutron cross section libraries and the calculational method for fusion neutronics. Energy spectra of scalar flux in the assembly above 0.8 MeV were measured at the positions of 8 to 88 cm in depth using a small spherical NE213 liquid scintillator with an unfolding code FORIST. The measured spectra were analyzed with the two-dimensional transport code DOT3.5 by using three multigroup cross section libraries originated from the ENDF/B-III, -IV, and JENDL-2 and -3PR1 nuclear data files. The calculated spectra with the 42-group library from ENDF/B-III are not consistent with the measured ones in shape. Calculation with the 125-group library from ENDF/B-IV overestimates the measured spectra in the whole energy region at every position, because of the uncertainty in the non-elastic cross section for Cr and the elastic and non-elastic cross sections for Fe. The best agreement is obtained between the measurement and the calculation using the 125-group library from JENDL-2 and -3PR1.