A HZSM-5 supported Ag nanoparticles catalyst, which has a Si/Al ratio of 1500, is synthesized by a simple impregnation method for soot oxidation. Ultrafine Ag nanoparticles (NPs) of 1-2 nm in diameter were well dispersed and stable without any aggregation up to 800 °C, confirmed by high resolution scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and high temperature X-ray diffraction. This catalyst exhibited high intrinsic soot oxidation activity and Tmax (peak temperature of DTA curves) values were 362 °C and 533 °C under tight contact and loose contact conditions, respectively. Furthermore, the physical and chemical states of Ag species before and during soot oxidation were examined by in situ X-ray absorption spectroscopy. Two kinds of Ag species (ZO-Ag+ and Ag NPs) were found in this catalyst; they were reduced and crystallized after heating over 150 °C. The Ag NPs remained stable without any aggregation even after 800 °C, which means the Ag/HZSM-5 catalyst can be well regenerated after high temperature treatment, ensuring good repeatability of soot oxidation performance.