The mutual interdependence of plants and arbuscular mycorrhizal fungi (AMF) is important in carbon and mineral nutrient exchange. However, an understanding of how AMF community assemblies vary in different forests and the underlying factors regulating AMF diversity in native tropical forests is largely unknown. We explored the AMF community assembly and the underlying factors regulating AMF diversity in a young (YF) and an old-growth forest (OF) in a tropical area. The results showed that a total of 53 AMF phylogroups (virtual taxa, VTs) were detected, 38±1 in the OF and 34±1 in the YF through high-throughput sequencing of 18S rDNA, and AMF community composition was significantly different between the two forests. A structural equation model showed that the forest traits indirectly influenced AMF diversity via the plant community, soil properties and microbes, which explained 44.2% of the total observed variation in AMF diversity. Plant diversity and biomass were the strongest predictors of AMF diversity, indicating that AMF diversity was dominantly regulated by biotic factors at our study sites. Our study indicated that forest community traits have a predictable effect on the AMF community; plant community traits and soil properties are particularly important for determining AMF diversity in tropical forests.