One of the main components that should be in place to control nuclear materials and their applications is a robust measurement system. The function of this system is not only to verify the correctness of the information regarding nuclear materials but it must be capable to characterize the unknown materials as well. The material's chemical and physical attributes concluded from the characterization process are needed to determine the intended use and origin of such materials. Thus, the characterization of nuclear materials is a key issue and equally important in nuclear safeguards (for inventory verification) and in nuclear security (for forensics attribution). Herein, an absolute spectrometric non-destructive analysis (NDA) method was used to characterize some plutonium-bearing materials. The gamma-ray spectra from fifteen samples were obtained using a high purity germanium detector. A portable HPGe-based radionuclide (Falcon 5000) with the In-Situ Object Counting System (ISOCS) which is based on a pre-efficiency calibrated high-resolution gamma-spectrometer in combination with analysis software was applied. The isotopic composition of the assayed samples was estimated from the direct measurement of the activities of Pu isotopes as well as Am, Pa, and U isotopes. The obtained results have shown that the proposed spectrometric method can be successfully used to characterize Pu-bearing samples for nuclear forensic purposes.