Abstract: On the basis of the decay law of fission products and the γ peak counts mea-sured by HPGe detector, the formula of calculating the total numbers of fission productsat zero time was deduced. Taking binary fission of ~(239)Pu as an example, the numbers ofgas fission products ~(85)Kr~m, ~(87)Kr,~(88)Kr, ~(133)Xe~m, ~(133)Xe,~(135)Xe~m, ~(135)Xe and ~(138)Xe as a func-tion of decay time were calculated, and the gamma ray count rates at the distance of 25cm from a HPGe detector with the detection efficiency of 60% were evaluated.