Abstract: Severe accident release categories of a nuclear power plant of third generation were introduced. The release characteristics to the environment were calculated using MAAP code for those release categories and its severe accident sequences which can result in the large release of radioactive materials. On this base, several nuclides which contribute the most personnel dose were selected. This paper calculated the cumulative activity released to the environment and whole body and thyroid dose of 500 m site boundary with and without considering the decay of nuclides. The effect of decay on the results was analyzed. All these analysis can provide some reference for the improvement of severe accident simulation program. The effect of decay on the cumulative activity of fission products released to the environment is related to the radionuclide half-life and the release time of the fission products after the accidents. From the off-site dose analysis, the effect of decay on whole body dose is more obvious than that on the thyroid dose.