This paper deals with the influences of herbicide mefenacet on soil respiration and the activities of dehydrogenase,catalase,urease,proteinase and phosphorylase in paddy soil.The results showed that after application of mefenacet with concentration of 0.067,0.100,0.133,0.200 and 0.267 g g^-1 dried soil,soil respiration was stimulated at the beginning but reduced slightly afterwards,and the activity of dehydrogenase was enhanced in paddy soil.The soil catalase was less active than dehydrogenase,even inhibited within the first two weeks.The soil urease was strongly depressed and the activity of proteinase took on the slight inhibition within the whole period of experiment.The activity of phosphorylase,however,was raised with the increase of mefenacet concentrations within the first 18 days.Mefenacet had lower effects on soil respiration and enzyme activities than butachlor,an old generation of herbicide.