Abstract: In order to investigate the mechanism of bioremediation of petroleum hydrocarbon-contaminated soil by composting, an experiment was conducted with bacteria agent and mature chicken manure as amendment. We studied the kinetics of petroleum hydrocarbon degradation and the diversity of microbial community during the bioremediation of petroleum hydrocarbon-contaminated soil by composting with different concentrations. The concentrations included 5 000 mg/kg (T1), 10 000 mg/kg (T2) and 50 000 mg/kg (T3). The results showed that biodegradation of petroleum hydrocarbon followed the first-order model during composting. The constants of biodegradation rate in 3 treatments respectively were 0.012, 0.094 and 0.050/d. The half-life period was 6.79 d in T1 treatment, 7.37 d in T2 treatment and 13.86 d in T3 treatment. The average degradation rate was 112.08 mg/(kg·d) in T1 treatment, 230.05 mg/(kg·d) in T2 treatment and 887.93 mg/(kg·d) in T3 treatment during composting. This indicated that the average degradation rate increased with the increase in the petroleum hydrocarbon concentration. The average well-color development (AWCD) and use of carbon sources (except aromatic compounds) increased during the composting process, and reached the peak at the end of composting. There was a sharp rise in AWCD at the beginning of composting. This phenomenon could be easily explained by the fact that the total activity of soil microbial community increased significantly in the early of the process, while the use of carbon sources rose. The value of AWCD and the use of carbon sources in T3 were significantly higher than that in T2 and T3 at the end of composting. This demonstrated that there were dominant microbial consortia in the treatment with higher petroleum hydrocarbon concentration, and the dominant microbial consortia raised the total activity of soil microbial community and the use of carbon source. The dominant microbial consortia were metabolism communities of polymers and carbohydrates in composting process. The principal component analysis results revealed that there was a significant difference in soil microbial community structure among 3 treatments and the difference was mostly related to the use of carbohydrates and carboxylic acids. The microbial community diversity, as indicated by Shannon and McIntosh, increased during the composting process, and reached the peak at the end of stage. The values of Shannon and McIntosh in T3 were 0.21% and 17.64% higher than those in T1 respectively, and the differences were significant at 0.05 level (P<0.05). Simpson reached the maximum in middle stage. The value of Simpson in T1 was 2.12% and 9.44% higher than that in T2 and T3 respectively (P>0.05). This phenomenon was likely due to the stimulating effect of lower concentration of petroleum hydrocarbon on the growth of the dominant microbial consortia. However, the structure of soil microbial community in 3 treatments had no significant difference. The seed germination index (SGI) reached the maximum at the end of composting. Compared with the first stage of composting, the SGI in 3 treatments increased respectively by 18.26%, 20.42% and 36.41%. This suggested that bioremediation of petroleum hydrocarbon-contaminated soil by composting had a high effect for improving soil health. The results can provide a reference and theoretical basis for the application of bioremediation in petroleum hydrocarbon-contaminated soil in the Loess Plateau by composting with different concentrations.