Anaerobic digestion produce considerable amount of digestates which may not meet the requirements for direct land spreading. Aerobic treatment provides an appropriate post-treatment method to deal with the increasing amount of digestates. In this work, we studied the nitrogen dynamic and the evolution of groups of microorganisms during aerobic treatment of digested sludge. The oxygen consumption concentrated in first 10 days which corresponded to the intense biodegradation of easily biodegradable materials. The mineralization of organic nitrogen in the digestates followed the intense biodegradation and contributed to considerable ammonia emissions. The ammonia oxidation flow decreased to around zero during the intense biodegradation phase and reestablished during the maturation phase. It contributed to the disappearance of ammonia emissions and a sharp decrease of stored ammoniacal nitrogen in compost. The low content of available carbon in the digestates probably limited the immobilization of ammoniacal nitrogen and the development of the total biomass of microorganisms. The number of ammonia-oxidizing bacteria decreased in first 5 days treatment but exhibited a significant development during the maturation phase. The development of ammonia-oxidizing archaea was not significant. Statistic correlation analysis indicates that the ammonia-oxidizing bacteria instead of the ammonia-oxidizing archaea were responsible for nitrification during the treatment of digested sludge (Sig. 0.01). Our results suggest that the length of composting process should be controlled to meet the needs of agriculture. It should ensure the consumption of excessive ammoniacal nitrogen but not reduce the fertilizer value of compost.