Nuclear characterization of the spent nuclear fuel in a reactor core is essential, especially in case of severe accidents. The radionuclide inventory and its activity can assist in the management of spent fuel handling, transport or reprocessing.In this paper, the core of Fukushima Daiichi Unit-1(FD-U1) accident was modeled using the Monte Carlo code (MCNPX 2.7) linked to the depletion calculation code CINDER'90 and ENDF/B-VII.0 cross section data library. The isotopic inventory and the activity of the radionuclides for the burned fuel were calculated. The input to the code depends on the previous evolution of the reactor core configurations, dimensions and material of the fuel assemblies, initial uranium enrichment, fuel burn-up and reactor core operational history.The calculations were validated with experimental measurements which were carried out by the Japan Nuclear Energy Safety Organization (JNES) and verified with published results using ORIGEN2-code by Japan Atomic Energy Agency (JAEA). The validation and verification results were in good agreement.The masses, activities, specific activities, half-lives and decay schemes for the actinides and fission products were calculated at the time of the accident and after 50 years cooling time. The calculations showed that, total activity of the burned fuel in the core at the time of the accident was 9.86E+19Bq and after 50 years was 1.89E+17Bq and the higher inventory concentration in the fuel was dominated by the trans-uranic elements. Also, the specific activity in the core at the time of the accident and after 50 years cooling time was found to be 1.84E+15Bq/g and 5.86E+12Bq/g, respectively. These calculations are required for nuclear characterization of the corium and in the estimation of the radiological consequences of the source term in the environment. Also, the results can support the recovery program for Fukushima Daiichi-Unit-1.