Abstract: The paper describes three determination methods of alpha activity ratio used at our laboratory,namely geometric progression method, extrapolation method and curve fitting method. The validity ofthe geometric progression method is demonstrated in the present work. The ~(238)Pu/ (~(239)Pu+~(240)Pu) evalu-ated by extrapolation method is compared with that by geometric progression method. Also, the ~(238)Pu/(~(239)Pu+~(240)Pu) evaluated by curve fitting method method is compared with that by geometric progres-sion.