Abstract: A computer code using recursive algorithm searching the paths of the decay chains was written to calculate the activity and the energy spectra at any time of one or one group of fissile products from a sample of fissile nuclides irradiated by neutrons. The fission delayed gamma spectra of ²³⁹Pu irradiated by pool reactor neutrons were calculated. The MCNP code was used to simulate the energy responses of HPGe detector to fission delayed gamma rays. The results of the simulation and calculation can be used as a guide to the measurements of fissile yields of nuclear materials and other relative research works.