Rational nitrogen (N) application can greatly increase rice (Oryza sativa L.) yield. However, excessive N input can lead not only to low N use efficiency (NUE) but also to severe environmental pollution. Reducing N application rate with a higher planting density (RNHD) is recommended to maintain rice yield and improve NUE. The effects of RNHD on fertilizer N fate and rice root growth traits remain unclear. We accordingly conducted a two-year field experiment to investigate the influence of RNHD on rice yield, fertilizer 15N fate, and root growth in a double-rice cropping system in China. In comparison with the conventional practice of high N application with sparse planting, RNHD resulted in similar yield and biomass production as well as plant N uptake. RNHD increased agronomic NUEs by 23.3％–31.9％(P<0.05) and N recovery efficiency by 17.4％–24.1％(P<0.05). RNHD increased fertilizer 15N recovery rate by 14.5％–34.7％(P<0.05), but reduced 15N retention rate by 9.2％–12.0％(P<0.05). Although a reduced N rate led to significantly reduced root length, surface area, volume, and biomass, these root traits were sig-nificantly increased by higher planting density. RNHD did not affect these root morphological traits and reduced activities of nitrate reductase (NR) and glutamine synthetase (GS) only at tillering stage. Plant N uptake was significantly positively correlated with these root traits, but not correlated with NR and GS activities. Together, these findings show that reducing N application with dense planting can lead to high plant N uptake by maintaining rice root growth and thus increase NUE.