AM (arbuscular mycorrhizal) fungi are ubiquitous in soils of almost all ecosystems, and normally form symbiotic relationship with root systems of over 80% territorial plant species. Mainly based on their positive effect on host plant mineral nutrients, AM fungi can influence the community structure of the plant ecosystem. An increased diversity index of grass community in a microplot experiment has been reported. We hypothesize that AM fungi may affect the formation of dominant and subordinate species in a given plant community due to the differences in mycorrhizal dependence. In present research, the growth response and nutrient uptake of dominant and subordinate plant species which were growing in two subtropical grassland ecosystems to indigenous AM fungi or to inoculation of exogenous fungus, Glomus mosseae, were investigated. Results showed that the mycorrhizal dependencies of the dominant species in two grasslands, Ageratum conyzoides and Paspalum conjugatum, on the indigenous AM fungi were 41.5% and 77.4%, respectively, much higher than those of the subordinate species, Cyperus difformis and Polygonum barbatum (16.0% and 7.9%). Their mycorrhizal dependencies (MD) on Glomus mosseae changed. The MDs of A. conyzoides, P. conjugatum, C. difformis and P. barbatum were 79.6%, 44.2%, 74.1% and 24.9%, respectively. This indicated that indigenous fungal species were one of the mechanisms responsible for the formation of dominant and subordinate species, and that inoculation of exogenous fungal species could change the community structure formed in symbiosis with the indigenous fungal species. Analysis of plant P nutrition showed that the growth promotion of dominant and subordinate species by AM fungi were highly related to the increase of P uptake by AM fungi, suggesting that increased nutrient uptake by AM fungi was one of mechanisms affecting plant community structure.