Abstract: Effectiveness of ozone degrading dimethoate in solution and its affecting factors were studied.The results showed that when the initial concentration of ozone was 10 mg·L^-1,80% of the dimethoate could be degraded within 5 min.After that,however,the degradation did not significantly increase with the time.The mechanism of ozone reacting with dimethoate was investigated by adding radical scavengers such as bicarbonate and tert-butanol.The results appeared to confirm that the degradation of dimethoate by ozone in the experiment condition followed the mechanism of molecular reaction.At the same time,through GC-MS analysis,omethoate was still found in the water treated with ozone.This shows that harmful byproduct will result from inadequate treatment with ozone.