A greenhouse experiment was conducted to study the effects of simulated nitrogen deposition on the growth and competition of C4 weedy species (Echinochloa crusgalli var. mitis L. ) and C3 upland rice (Oryza sativa L.) under day/night temperature of 35 degrees C/25 degrees C and 30 degrees C/20 degrees C. The results showed that under day/night temperature 35 degrees C/25 degrees C, 4.0 g x m(-2) x a(-1) N deposition enhanced the shoot biomass of E. crusgalli and O. sativa by 29.18% and 27.80%, respectively, compared with control. The N and P uptake by E. crusgalli increased by 87.33% and 49.73%, respectively, but no change was found for O. sativa. Under 30 degrees C/20 degrees C, three N depositions (2, 4 and 6 g x m(-1) x a(-1) ) enhanced the shoot biomass, tiller number, and shoot N and P uptake of E. crusgalli by 48.99%, 72.68% and 36.18%, 111.11%, 122.22% and 144.44%, 108.88%, 129.22% and 134.29%, and 16.53%, 65.05% and 22.47%, respectively, but no changes were observed for O. sativa. Nitrogen deposition significantly increased the shoot biomass ratio of E. crusgalli to O. sativa under 30 degrees C/20 degrees C, but had no impact on this ratio under 35 degrees C/25 degrees C. It was suggested that N deposition could enhance the competition of E. crusgalli over O. sativa, especially under lower air temperature.