Quality of (U,Pu)O₂ mixed oxide (MOX) fuels for fast reactor is judged by its fissile worth; quantified as Pu²³⁹ equivalence which is governed by the isotopic composition of PuO₂ used for fuel fabrication and the concentration of PuO₂. An experimental method for directly estimating Pu²³⁹ equivalence of MOX fuel pins for the upcoming Prototype Fast Breeder Reactor (PFBR) was attempted using passive gamma scanning (PGS) in this work. A combination method of theoretical and experimental estimations was used to study the dependence of deterioration in fissile worth of stored fuel pins with corresponding change in their PGS responses. This is a novel and effective method to estimate the updated value of fissile worth at the time of reactor loading after long duration of storage after fabrication of fuel pins.