Abstract: Soil fumigants have been comprehensively used to control nematodes, weeds, soil-borne pathogens and insects in high value (cash) crop plantations. Though ideal pesticides are toxic only to target organisms, fumigants are a class of pesticides with broad biocidal ability thus affecting many non-target soil organisms, including nitrifying and denitrifying bacteria. It may results in changed soil nitrification and denitrification. The aim of this study was to explore the proper application measures of fumigants by quantifying the effects of different soil fumigants on soil nitrification and denitrification in laboratory and field conditions. To investigate the effects of different soil fumigants on soil nitrogen transformation processes of nitrification and denitrification, chemical analysis and degeneration gradient gel electrophoresis technique (DGGE) were used. Soils fumigated with metham-sodium, dazomet, methyl bromide and sulfuryl fluoride for a period of 100 d were sampled and the effects of fumigation on soil nitrification and denitrification activities determined along with the community structures of amoA nitrification bacteria and nirS denitrifying bacteria. The results showed that compared with the control, no significant differences existed in nitrification activity of soils fumigated with metham-sodium, dazomet, sulfuryl fluoride. However, nitrification activity of soils fumigated with methyl bromide decreased by 13.19%, which was significant (P < 0.05) compared with the control. No significant differences existed among the four fumigants in terms of observed soil nitrification activity. There was also no significant difference in soil denitrification activity among the four fumigants and the control. Diversity index and evenness index of amoA nitrifying bacteria community of soil fumigated with methyl bromide were significantly lower than those of soils fumigated with the other three fumigants and the control. Richness index was not significantly different among all the treatments. The three indexes were not significantly different among soils treated with metham-sodium, dazomet, sulfuryl fluoride and the control. Diversity index, evenness index and richness index of nirS denitrifying bacteria community were significantly different among the four fumigants, but not significantly different from those of the control. The research results showed that the inhibition effect of methyl bromide on soil nitrification activity was achieved through the inhibition of the diversity of amoA nitrifying bacteria community. The other three fumigants had no significant effect on soil nitrification activity. Denitrification activity of soils was not significantly affected by the four fumigants.