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The effect of high-energy ball milling on the structure of nanometer sized amorphous ceramics, a-Si3N4 and a-Si–N–C, respectively, has been investigated. At high milling intensity, a-Si3N4 may rapidly crystallize into a mixture of α-Si3N4 and β-Si3N4 after the initial 4 s of milling whereas no structural changes were observed at low milling intensity. For a-Si–N–C, mechanical milling does not cause structural changes at both low and high intensity. It was found that extension of mechanical milling of these hard ceramics can introduce large volume fractions of contamination fragments from the milling media. We conclude that the observed structural changes occurring in a-Si3N4 may be due to mechanical effect, rather than local heating and/or impurity effect.