Simultaneous nitrification and denitrification (SND) is a new wastewater treatment process for biological nitrogen removal, which shows some significant advantages compared with conventional biological nitrogen removal processes. The SND process in a fixed bed biofilm reactor with microbubble aeration was investigated in this study. The removal efficiencies of COD and nitrogen were determined under different operational conditions and the functional bacterial populations for nitrogen removal in the biofilm were detected. The results showed that efficient SND process could be achieved in the biofilm reactor with microbubble aeration. The SND could be improved at lower dissolved oxygen (DO) concentration and larger porosity of packing bed when the COD loading rate and C/N ratio were increased. The removal efficiencies of COD and total nitrogen (TN) were 97.6% and 70.2%, respectively, at a COD loading rate of 0.86 kg x (m3 x d)(-1), a TN loading rate of 0.10 kg x (m3 x d)(-1), and a packing bed porosity of 81%, indicating the simultaneous efficient removal of COD and TN. Under these conditions, the oxygen utilization efficiency reached as high as 91.8% due to the enhanced oxygen mass transfer by microbubble aeration. In addition, the biofilm activity and the abundance of nitrifiers and denitrifiers were consistent with the removal capacity of COD, ammonia and TN under different operational conditions.