A pot experiment was conducted to study the regulation effects of arbuscular mycorrhizal fungi (AMF) on the interactions between Echinochloa crus-galli var. mitis L. and Oryza sativa L. under enhanced N supply (4.0 g N x m(-2) x a(-1)). The results showed that under monoculture condition, the AMF colonization on E. crus-galli increased but that on O. sativa decreased. In the treatments with and without AMF inoculation, upland rice biomass and its P and N uptake increased by 42.35% and 13.48%, 4.07% and 2.55%, and 30.35% and 62.09%, respectively, and barnyard grass biomass and its P and N uptake increased by 20.24% and 15.65%, 3.88% and 4.06%, and 15.10% and 30.35%, respectively. Under mixed cropping, the AMF colonization on E. crus-galli increased but that on O. sativa had little change. In the treatments with and without AMF inoculation, the biomass ratio of O. sativa to E. crus-galli decreased, but N uptake ratio changed a little. The P uptake ratio of O. sativa to E. crus-galli increased in treatment without AMF inoculation but decreased in treatment with AMF inoculation. It was suggested that AMF tended to enhance the competition of E. crus-galli to O. sativa under enhanced N supply.