Abstract: Transgenic Bt cotton has been released into the environment commercially for over 10 years in China,and its influence on soil microbes is an important part of biosafety research.To monitor impact of long-term cultivation of transgenic Bt cotton on diversity of soil microbial community,one cotton field where non-transgenic had been planted all along and other two cotton fields where transgenic Bt cotton had been planted for seven and ten years,respectively,were selected and soil samples were taken from these fields at four different cotton growth stages,namely,seedling,budding,boll forming and boll opening stage during 2007 and 2008.Results show that the populations of bacteria,fungi,azotobacter,denitrobacteria,nitrosobacteria and the diversity indices of microbes have a similar variation pattern during the cotton growing period in three cotton fields and their populations peaked at the budding stage.There was no significant difference in population and diversity index of soil microbes between the two transgenic Bt cotton fields different in cultivation history,but significant seasonal variation of the populations and diversity indices of the microbes were observed in all the three cotton fields.