Background: Selective oxidation of chemical warfare agent (CWA) simulants including the 2-chloroethyl ethyl sulfide (2-CEES) and other analogs through a green method in the presence of keplerate Cu20-Polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- supported on magnetic nanoparticles was investigated. Keplerate nano-polyoxometalates, a new class of polyoxometalates which commonly known as “Giant”, are desirable materials, used for rapid selective oxidation bis (2 chloroethyl) sulfide to sulfoxid due to the toxic nature of mustard gas, the less toxic but structurally similar analogue 2-chloroethyl ethyl sulfide (2-CEES) is commonly used to study this reaction.Methods: The was catalyst characterized by different techniques, such as XRD, FT-IR, TGA, ICP-AES, TEM and SEM. Herein we reported the first used of this Nano-catalyst as selective and recoverable heterogeneous catalysts for the sulfoxidation of the CWA stimulant 2-CEES and other analogs using H2O2 as the oxidant under mild conditions. It was observed that the [WPCu@ASMNP] shows the highest catalytic activity on sulfoxidation of thioether mustard (HD) analogs with water as green solvent. Influence of various reaction parameters on the sulfoxidation reaction was studied. The catalyst was reused up to 5 times after separation with an external magnet without observable loss of activity and selectivity (According to the results of the FT-IR and XRD). Finally, plausible mechanism for the sulfoxidation thioethers mustard is discussed. Results: After optimizing the reaction conditions, oxidation yields of ten representative thioethers, two diaryl sulfide, two aryl alkyl sulfide and six dialkyl sulfides using H2O2 in the presence of [WPCu@ASMNP] at 25°C was obtained. After reaction completion, the catalyst was easily separated by using an external magnet and tested for five subsequent runs under the optimized reaction conditions. The yield and selectivity of the corresponding sulfoxide was found to be almost similar during these experiments. The recovered catalyst after five cycles was characterized by using FTIR and XRD which indicated results in good agreement with the fresh one, confirming the stability as well as structural integrity of the catalyst. Conclusion: We describe an environmentally friendly protocol for the oxidation of thioether mustard (HD) analogs into sulfoxide at 25°C using 30% aqueous H2O2 under mechanical stirring. In the oxidation system we also observed only one product (sulfoxide) without sulfone. The transformation worked well with both the liquid and solid sulfides in spite of a heterogeneous reaction mixture, and no difficulties with stirring were observed. The FT-IR and XRD of the catalyst show retention of [WPCu@ASMNP] after reaction. At the moment, we are investigating the activity of other giant POMs in oxidation chemical warfare agents. We think that these results open a whole new family of active catalysts in oxidation CWAs.