Abstract: A comparison of the degradation kinetics of imazapyr was conducted in four typical soils under sterilized and non-sterilized conditions, and the rates of degradation by microorganisms preliminarily investigated. The results showed that the half-life values ranged from 29 to 44 days in non-sterilized soils and from 81 to 134 days in sterilized soils. The degradation rate constants in non-sterilized soils were found to substantially increase, (1.3)～(3.4) times higher than that in sterilized soils. The theoretical half-life of imazapyr by microbial degradation in powder soil was about 48 days, nearly 3 times of that under sterilized conditions. The preceding conclusions fully proved that the intrinsic microbes in soils play an important role in the degradation of imazapyr. Four metabolic products, which molecular weights were 219, 222, 201 and 149, respectively, were analyzed on the basis of the spectral data of LC-ESI-MS. The principal degradation pathway for imazapyr in soils was proposed as follows: the incorporation of imazapyr into NH+4, yielding a stable isopropylammonium salt; then further decarboxylation, demethylation, loss of isopropyl group, cleavage and rearrangement of imidazolinone ring and so on.