Abstract: The effect of three arbuscular mycorrhizal fungus（AMF） on growth and key enzyme activities of nitrogen assimilation was measured in maize under three different ratios of NH4+∶NO3- using perlite as the substance,. The results showed that, nitrate reductase activity on maize leave was enhanced by Glomus intraradices and Glomus mosseae inoculation when the ratios of NH4+∶NO3ˉ were 3∶1 and 1∶3, respectively. The activity of glutamine synthetase（GS） of maize leaf was not affected significantly by AMF, but all the three AMF inoculation enhanced GS activity of roots and the ratio of ammonium assimilated in roots at NH4+∶NO3ˉ=3∶1. Under the conditions of high ammonium content, AMF had a significant impact upon plant growth promotion, and root GS activity enhancement, as the result, AMF play an more important role in ammonium utilization of host plants.