Soft X-rays assisted modification of polymeric nanocomposites containing metal nanoparticles has been performed and radiation induced changes of their physical and chemical properties have been investigated. Thin composite layers were formed on polished glass substrates applying spin-coating technique for composite deposition from solution containing metal powder additives. Synthesis of metal nanoparticles was performed directly in the deposited PMMA layer applying photocatalytic method (UV irradiation), which was followed by sample irradiation with soft X-rays. The surface morphology, composition and microstructure of samples were assessed using scanning electron microscope, energy dispersive X-ray spectroscopy and optical microscopy. Also UV-VIS spectrometry and FTIR spectrometry were applied for characterization of nanocomposites. It was found that the soft X-ray irradiation contributed to the formation of new Ag particles and their agglomeration into larger clusters within the polymer matrix causing changes of structural and optical properties of polymeric nanocomposites. Mechanisms of low-dose rate X-ray radiation induced structural changes in polymeric composites, processes promoting formation of new particles in polymers; particles growth and clusterization within polymer matrix have been discussed analyzing changes of physical and chemical properties of nanocomposites before and after their X- ray irradiation.