Abstract: The effective capture-to-fission of ~(235)U (α_5) could be obtained by α_5= FBu (%) induced from the correlative curves of α_5 versus burn-up Bu (%). The burn-up values were calculated from ~(236)U/~(235) U ratio values before and after irradiation in the irradiated nuclear fuels and the function α_5= FBu (%) by digital approximation. The burn-up values of five different kinds of reactors with 10 % ~(235)U have been calculated. The burn-up values calculated by this method and indicator-~(148)Nd-spent-urani um concentration method agreed with in ±3%。