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Silicon oxynitride nanopowders were successfully prepared by thermal explosion of the Si and SiO 2 mixed powders with the different particle sizes in various molar ratio (x / y = 2.0, 1.5) under N 2 , supplemented by the subsequent gravity separation strategy. The effects of Si/SiO 2 molar ratio, preheating temperature and insulation time on the phase composition of the as-oxynitrided products in the thermal explosion strategy were investigated. Meanwhile, the effects of sedimentation time on the phase composition and oxygen content of the as-obtained products in the gravity separation strategy were also discussed. It is shown that the by-products are absent in the as-oxynitrided products with a 1.5:1 M ratio. Prolonging the insulation time or preheating temperature is beneficial to improve the content of Si 2 N 2 O phase with a 2.0:1 M ratio. All the by-product phases are disappeared in the as-oxynitrided products with a 2.0:1 M ratio after sedimentation treatment of 120 s. The as-obtained silicon oxynitride nanopowders with 58% of recovery and 16.2% of oxygen content are close to the theoretical stoichiometric ratio of Si 2 N 2 O. The preparation mechanism of silicon oxynitride nanopowders was proposed. [ABSTRACT FROM AUTHOR]