Abstract: The behaviors of fission product were analyzed by using integral safety analyse code for nuclear power plant of 900 MW in the case of severe accident induced by large break loss of coolant accident. The behaviors including release, transport and distribution of fission product were analyzed, and the source terms into the environment were also calculated. It can be concluded that almost all of the inert gases release into the environment, the fraction of volatile radionuclide released into the environment is 10^-3 quantity, and the fraction of nonvolatile radionuclide released into the environment is 10^-6-10^-8 quantity. The results are useful for the assessment of the accident consequences.