Short term effect of mefenacet, an acetanilide herbicide, on soil microbial communities in paddy soil was stud ied. The Biolog method (a kind of colour development assay) was used to determine substrate utilization potential of microbial communities for assessment of biological communities. The results show that functional diversity of the microbial communities decreased, and capability of the microbes of utilizing substrate of simple carbon sources reduced in mefenacet-polluted soils compared with those in non polluted soil. However, this effect was transient, and functional diversity of the microbial communities in mefenacet polluted soils came back at the end of the experiment. Supplementation of strain Y1 had a positive effect on metabolic potential of the soil microbial communities. A kinetic method was presented for characterizing the physiological state of microorganisms capable of carrying out specific metabolic function in paddy soils. Test well dye reduction was non linear in microplates inoculated with duration of the incubation, and the color development curve was generally sigmoidal in shape. Test well OD values over time suggested a kinetic model based on the density-dependent logistic growth equation. The kinetic parameters generated by the model can be used as surrogates for single-time-point data in constructing carbon source utilization patterns. And a and x₀ were more sensitive to indicate the effects of mefenacet and strain Y1 on functional diversity of the microbial communities. Shannon index (H) was more sensitive than the index of microbial community richness (S) to show the microbiological change in contaminated environment.