A one-group cross section data base used by the ORIGEN2 computer code to simulate the depletion, buildup and decay of radionuclides in research reactor was developed. For this, ENDF/B-IV or -V data was processed using the NJOY code system into 69-group data. The burn-up-dependent weighting spectra were calculated with the WIMS-KAERI code, and then the 69-group data were collapsed to one-group using the spectra. The ORIGEN2 depletion calculations for the KMRR fuel were performed using an old PWR and the new data base. By comparing these results to the WIMS-KAERI calculations, it is seen that the results of actinide composition calculated by the ORIGEN2 with the new data base turn out to be in an excellent agreement with the WIMS-KAERI results in the range up to 120 GWD/MTIHM burnup. (Author).