The potential of mechanochemical treatment (MC) to degrade PCDD/Fs contained in fly ash was tested via grounding with and without calcium oxide (CaO) under atmospheric pressure. Three types of fly ash collected from medical waste incineration were compared, originating either from rotary kiln fluidized bed multi-stage incinerator using activated carbon spray (FA1, FA2), or a simple stoker incinerator without activated carbon spray (FA3). In test 1: CaO to FA1 mixed at ratio of 6-60% was milled at rotational speed of 350 rpm; in test 11: FA2 and FA3 without CaO were milled at rotational speed of 400 rpm. The duration of the tests was 2 h. The results from the present study indicate that (1) under two test conditions of with and without CaO, PCDD/Fs contained in real fly ash both can be degraded by mechanochemical treatment, (2) under condition of blending with CaO, the degradation efficiency of PCDD/Fs increased with increasing ratio of CaO, (3) the degradation efficiency of PCDD/Fs may increase with rotational speed increasing and (4) the destruction and dechlorination are major mechanism for PCDD/Fs degradation. These results show that mechanochemical treatment is a high potential technology for PCDD/Fs degradation in fly ash. (C) 2007 Published by Elsevier B.V.