An iterative correction method based on gamma attenuation in matrix material and self-absorption in plutonium has been developed for the nondestructive gamma assay of plutonium waste. In the process, the equivalent atomic number of the matrix material and the lump radius of plutonium are estimated. These estimations can be used for the detection against possible covert diversion of plutonium. Both homogeneous and heterogeneous samples were tested and consistent results were obtained. Specially shielded lump plutonium samples canned in the waste drums were tested and the contents of those samples were adequately disclosed.